Add documentation about initial_value regarding mock_hw (backport #1352)

[mergify]

This should fix #1351. I added documentation on the mock_hardware page and on the components overview page in the example URDF.

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This is an automatic backport of pull request #1352 done by Mergify.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.37%. Comparing base (c016f14) to head (627abbd).
Report is 2 commits behind head on iron.

❗ Current head 627abbd differs from pull request most recent head 155bbcc. Consider uploading reports for the commit 155bbcc to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             iron    #1514   +/-   ##
=======================================
  Coverage   62.37%   62.37%           
=======================================
  Files          97       97           
  Lines       11059    11059           
  Branches     7948     7948           
=======================================
  Hits         6898     6898           
+ Misses        734      732    -2     
- Partials     3427     3429    +2     

Flag	Coverage Δ
unittests	62.37% <0.00%> (ø)

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see 1 file with indirect coverage changes