update and add more docs

[johnnychen94]

The docs are rewritten a bit to better demonstrate how this package is used with so many new features @jishnub have added.

The docs are updated using julia 1.6-DEV, so there're jldoctest warnings if you check the logs. Most of them are Matrix{Int64} <==> Array{Int64, 2} things.

preview: https://juliaarrays.github.io/OffsetArrays.jl/previews/PR145/

The workaround to conditional push preview requires github action as the CI platform, so I switched from travis to github action for docs deployment.

[codecov]

Codecov Report

Merging #145 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #145   +/-   ##
=======================================
  Coverage   93.60%   93.60%           
=======================================
  Files           2        2           
  Lines         250      250           
=======================================
  Hits          234      234           
  Misses         16       16           

Impacted Files	Coverage Δ
src/OffsetArrays.jl	94.17% <ø> (ø)
src/axes.jl	90.90% <ø> (ø)

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[jishnub]

The main page that describes the usage seems to use the earlier notation of Array{Float64,2}, whereas the developer page has now been updated to use Matrix{Float64}. Perhaps these should be consistent?

[johnnychen94]

The main page that describes the usage seems to use the earlier notation of Array{Float64,2}, whereas the developer page has now been updated to use Matrix{Float64}.

When jldoctest fails, it uses the actual output instead of the reference output; this in the docs will be updated automatically when Julia 1.6 is out 😃

[jishnub]

The docstring for IdOffsetRange needs to be updated, this triggered a build error in the docs

[johnnychen94]

There was a Documenter bug that if you run doctest multiple times, it simply gets skipped without warnings.

JuliaDocs/Documenter.jl#974

[jishnub]

Looks good