Add doxyfile (backport #279)

[mergify]

Needed for #278 on all branches for multiversion.

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This is an automatic backport of pull request #279 done by Mergify.

[mergify]

Cherry-pick of 481e732 has failed:

On branch mergify/bp/galactic/pr-279
Your branch is up to date with 'origin/galactic'.

You are currently cherry-picking commit 481e732.
  (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:
	new file:   doc/Doxyfile
	new file:   doc/api_list/mainpage.md
	new file:   images/logo_ros-controls_small.png

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   doc/api_list/api_list.rst
	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