Add API page (backport #268)

[mergify[bot]]

I'd like to include the API documentation of all packages, also for the utilities like control_toolbox etc.

I see two options:

1. Make a table and just link to docs.ros.org like here. It will be a bit more visible than now, but not searchable from control.ros.org
2. Let rosdoc2 create the pages manually and publish a selection on control.ros.org. Advantage (or maybe also disadvantage?): Everything can be found from within a single search prompt.

The majority was for option 1 during the WG meeting on April 10th. I changed it now to a simple table:

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This is an automatic backport of pull request #268 done by Mergify.

[mergify[bot]]

Cherry-pick of 159087e9e4f6b992aac3797c5eb875944fffbd37 has failed:

On branch mergify/bp/humble/pr-268
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit 159087e.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
    modified:   conf.py
    new file:   doc/api/api.rst

Unmerged paths:
  (use "git add <file>..." to mark resolution)
    both modified:   index.rst

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally