Documentation fixes for tf2_kdl (backport #869)

[mergify]

Generated-by: Portions of this commit may include code completion from github.copilot version 1.372.0 or later

Description

This PR updates tf2_kdl so that the python API shows in rosdoc2. Issues fixed:

• rosdoc2 looks to the existence of <buildtool_depend>ament_cmake_python</buildtool_depend> in package.xml to decide to build the python API documentation. This was missing. This dependency in theory is required for ament_python_install_package in CMakeLists.txt to be found. Probably the cmake builds worked anyway since dependencies of tf2_kdl already installed ament_cmake_python, but rosdoc2 does not rely on such transitive dependencies.
• package python3-pykdl results in rosdoc2 mocking 'pykdl' but the actual python import is PyKDL. Adding this to <doc_depend> is needed to overcome this.
• the python source is in a non-standard location, hence the addition to rosdoc2.yaml.
• The documentation directory was docs/source which is a non-standard location. Documentation was moved to the supported location doc/source. Most of the stuff there was out of date, so the only thing that survived was conf.py which is identical to other conf.py in this repo. Content in mainpage.dox was moved to the README instead.

Is this user-facing behavior change?

Ideally no, it should only affect documentation.. In some forms of building, the missing buildtool_depend>ament_cmake_python</buildtool_depend> could have resulted previously in the python code not building at all, but that apparently did not actually occur. In any case I assume you wanted the python API to build, and have its documentation exposed, which is what this PR provides.

Did you use Generative AI?

See header.

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This is an automatic backport of pull request #869 done by [Mergify](https://mergify.com).

[mergify]

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Changes to be committed:
	new file:   tf2_kdl/README.md
	renamed:    tf2_kdl/docs/source/conf.py -> tf2_kdl/doc/conf.py
	deleted:    tf2_kdl/docs/Doxyfile
	deleted:    tf2_kdl/docs/Makefile
	deleted:    tf2_kdl/docs/mainpage.dox
	deleted:    tf2_kdl/docs/make.bat
	deleted:    tf2_kdl/docs/source/index.rst

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   tf2_kdl/package.xml
	deleted by us:   tf2_kdl/rosdoc2.yaml

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