Fix doc warnings and make docs build strict

[KnutAM]

Currently, we have quite some doc warnings for missing / duplicate docstrings etc.
These should be fixed, but probably even better, we should enforce no such errors?

List of current errors (took from #889 CI which already fixes some), if fixing some, please check them off here

• Missing / duplicate docstrings (fixed in Add updated function names in devdocs #1036)
  • Ferrite.shape_gradient(::Interpolation, ::Vec, ::Int)
  • Ferrite.shape_gradient_and_value
  • Ferrite.shape_gradients!
  • Ferrite.shape_gradients_and_values!
  • Ferrite.shape_value(::Interpolation, ::Vec, ::Int)
• Filter docstring correctly (WIP in Fix doc warnings and make docs build strict #1039)
  • getrefdim(::Interpolation)
• Bib-issues

┌ Warning: The entry "tutorials/dg_heat_equation.md" in the Pages attribute of the @bibliography block on page src/tutorials/dg_heat_equation.md appears to be relative to "src". Starting with DocumenterCitations 1.3, names in `Pages` must be relative to the folder containing the file which contains the `@bibliography` block.
└ @ DocumenterCitations ~/.julia/packages/DocumenterCitations/OyiEp/src/expand_bibliography.jl:287
┌ Warning: The DOI field in bibtex entry "Mu:2014:IP" should not be a URL. Extracting "https://doi.org/10.1016/j.cam.2013.06.003" -> "10.1016/j.cam.2013.06.003".
└ @ DocumenterCitations ~/.julia/packages/DocumenterCitations/OyiEp/src/formatting.jl:388
┌ Warning: The DOI field in bibtex entry "SimMie:1992:act" should not be a URL. Extracting "https://doi.org/10.1016/0045-7825(92)90170-O" -> "10.1016/0045-7825(92)90170-O".
└ @ DocumenterCitations ~/.julia/packages/DocumenterCitations/OyiEp/src/formatting.jl:388
┌ Warning: The DOI field in bibtex entry "Mu:2014:IP" should not be a URL. Extracting "https://doi.org/10.1016/j.cam.2013.06.003" -> "10.1016/j.cam.2013.06.003".
└ @ DocumenterCitations ~/.julia/packages/DocumenterCitations/OyiEp/src/formatting.jl:388
┌ Warning: The entry "gallery/topology_optimization.md" in the Pages attribute of the @bibliography block on page src/gallery/topology_optimization.md appears to be relative to "src". Starting with DocumenterCitations 1.3, names in `Pages` must be relative to the folder containing the file which contains the `@bibliography` block.
└ @ DocumenterCitations ~/.julia/packages/DocumenterCitations/OyiEp/src/expand_bibliography.jl:287

• Link issues

 ┌ Warning: Cannot resolve @ref for md"[`write_projected`](@ref)" in src/reference/export.md.
│ - No docstring found in doc for binding `Ferrite.write_projected`.
│ - No docstring found in doc for binding `Main.write_projected`.
└ @ Documenter ~/.julia/packages/Documenter/qoyeC/src/utilities/utilities.jl:46
┌ Warning: Cannot resolve @ref for md"[Sparsity Pattern](@ref)" in src/topics/assembly.md.
│ - No docstring found in doc for binding `Base.-`.
└ @ Documenter ~/.julia/packages/Documenter/qoyeC/src/utilities/utilities.jl:46
┌ Warning: Cannot resolve @ref for md"[`InterpolationInfo`](@ref)" in src/devdocs/dofhandler.md.
│ - No docstring found in doc for binding `Main.InterpolationInfo`.
└ @ Documenter ~/.julia/packages/Documenter/qoyeC/src/utilities/utilities.jl:46
┌ Warning: Cannot resolve @ref for md"[`__close!`](@ref)" in src/devdocs/dofhandler.md.
│ - No docstring found in doc for binding `Main.__close!`.
└ @ Documenter ~/.julia/packages/Documenter/qoyeC/src/utilities/utilities.jl:46
┌ Warning: Cannot resolve @ref for md"[`_find_field(sdh::SubDofHandler, field_name::Symbol)`](@ref)" in src/devdocs/dofhandler.md.
│ - No docstring found in doc for binding `Ferrite._find_field`.
│ - No docstring found in doc for binding `Main._find_field`.
└ @ Documenter ~/.julia/packages/Documenter/qoyeC/src/utilities/utilities.jl:46
┌ Warning: Cannot resolve @ref for md"[`geometric_value`](@ref)" in src/devdocs/FEValues.md.
│ - No docstring found in doc for binding `Main.geometric_value`.
└ @ Documenter ~/.julia/packages/Documenter/qoyeC/src/utilities/utilities.jl:46
┌ Warning: Cannot resolve @ref for md"[facets](@ref Reference shapes)" in src/topics/boundary_conditions.md.
│ - Exception trying to find docref for `Reference shapes`: unable to parse the reference `Reference shapes` in src/topics/boundary_conditions.md.
└ @ Documenter ~/.julia/packages/Documenter/qoyeC/src/utilities/utilities.jl:46
┌ Warning: Cannot resolve @ref for md"[`create_sparsity_pattern`](@ref)" in src/reference/sparsity_pattern.md.
│ - No docstring found in doc for binding `Ferrite.create_sparsity_pattern`.
│ - No docstring found in doc for binding `Ferrite.create_sparsity_pattern`.
└ @ Documenter ~/.julia/packages/Documenter/qoyeC/src/utilities/utilities.jl:46
┌ Warning: Cannot resolve @ref for md"[`create_sparsity_pattern`](@ref)" in src/reference/sparsity_pattern.md.
│ - No docstring found in doc for binding `Ferrite.create_sparsity_pattern`.
│ - No docstring found in doc for binding `Ferrite.create_sparsity_pattern`.
└ @ Documenter ~/.julia/packages/Documenter/qoyeC/src/utilities/utilities.jl:46
┌ Warning: Cannot resolve @ref for md"[`vtk_cell_data_colors`](@ref)" in src/reference/grid.md.
│ - No docstring found in doc for binding `Ferrite.vtk_cell_data_colors`.
│ - No docstring found in doc for binding `Main.vtk_cell_data_colors`.
└ @ Documenter ~/.julia/packages/Documenter/qoyeC/src/utilities/utilities.jl:46
┌ Warning: invalid local link/image: file does not exist in src/topics/reference_shapes.md
│   link =
│    @ast MarkdownAST.Link("../devdocs/reference_cells/", "") do
│      MarkdownAST.Text("Developer documentation")
│    end
│    
└ @ Documenter ~/.julia/packages/Documenter/qoyeC/src/utilities/utilities.jl:46
[ Info: CheckDocument: running document checks.
┌ Warning: no footnotes found for '1' in src/topics/sparse_matrix.md.
└ @ Documenter ~/.julia/packages/Documenter/qoyeC/src/utilities/utilities.jl:46
┌ Warning: no footnotes found for '2' in src/topics/sparse_matrix.md.
└ @ Documenter ~/.julia/packages/Documenter/qoyeC/src/utilities/utilities.jl:46

[KristofferC]

These should be fixed, but probably even better, we should enforce no such errors?

Yes, definitely should aim for running with strict.