Custom components User Guide reorg

[droumis]

Migrates the Custom components User Guide into:

1. How To about Viewer components (combining existing components)
2. How To about ReactiveHTML (building from scratch recipes)
3. Background about ReactiveHTML (building from scratch, with all the discussion and details)

[codecov]

Codecov Report

Merging #4412 (be6aeef) into main (06ae142) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #4412      +/-   ##
==========================================
- Coverage   82.78%   82.78%   -0.01%     
==========================================
  Files         240      240              
  Lines       34822    34822              
==========================================
- Hits        28829    28826       -3     
- Misses       5993     5996       +3     

Flag	Coverage Δ
ui-tests	38.25% <ø> (+<0.01%)	⬆️
unitexamples-tests	73.79% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
panel/tests/util.py	89.32% <0.00%> (-1.95%)	⬇️
panel/io/state.py	70.29% <0.00%> (-0.35%)	⬇️
panel/util/__init__.py	87.11% <0.00%> (+0.44%)	⬆️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[philippjfr]

Looks great but I'd probably suggest we move the two new How To guides into a section about extending Panel.

[philippjfr]

Thanks @droumis!