Enable document generation using rosdoc2 for ament_python pkgs

[Yadunund]

Fix missing exec_depends in package.xmls and docstrings to generate documentation without any warnings using autodoc

Note: Without explictly casting HIDDEN_NODE_PREFIX to str, autodoc will throw an error as it cannot infer the type of this variable since dep modules are not imported.

[clalancette]

CI:

• Linux
• Linux-aarch64
• Windows