Update documentation

[urfeex]

• Add anchor to main page to be used by the assembled documentation
• Add github_url directives in order for the "Edit on GitHub" links to work properly in the assembled documentation.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 74.84%. Comparing base (daed243) to head (8d688b8). Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #309      +/-   ##
==========================================
- Coverage   75.10%   74.84%   -0.26%     
==========================================
  Files          86       86              
  Lines        3668     3666       -2     
  Branches      416      416              
==========================================
- Hits         2755     2744      -11     
- Misses        696      703       +7     
- Partials      217      219       +2     

Flag	Coverage Δ
start_ursim	67.88% <ø> (-0.75%)	:arrow_down:
ur20-latest	73.46% <ø> (-0.12%)	:arrow_down:
ur5-3.14.3	74.05% <ø> (-0.21%)	:arrow_down:
ur5e-10.7.0	68.00% <ø> (-0.15%)	:arrow_down:
ur5e-5.9.4	73.88% <ø> (ø)

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