Add custom rosdoc2 config for ros2_controllers metapackage (backport #1100)

[mergify]

rosdoc2 will be used by the build farm to publish API docs on http://docs.ros.org/en/rolling/p/ros2_control/ which is empty now.

• Add custom landing page for rosdoc2: logo, links to control.ros.org
• add rosdoc2 workflow, but only if the relevant files changed

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This is an automatic backport of pull request #1100 done by [Mergify](https://mergify.com).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.44%. Comparing base (8939bd4) to head (10d1887).

Additional details and impacted files

@@            Coverage Diff             @@
##             iron    #1143      +/-   ##
==========================================
- Coverage   86.46%   86.44%   -0.03%     
==========================================
  Files          86       86              
  Lines        7395     7397       +2     
  Branches      612      614       +2     
==========================================
  Hits         6394     6394              
  Misses        773      773              
- Partials      228      230       +2     

Flag	Coverage Δ
unittests	86.44% <ø> (-0.03%)	⬇️

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see 2 files with indirect coverage changes