Fix TOC in docs

[Bhavin-umatiya]

Cherry-pick changes from 7b2462a to fix the broken motion_primitive_controller TOC.

• Move README.md and userdoc.rst into doc/ subdirectory
• Fix path in controllers_index.rst
• Sort controller entries alphabetically
• Add proper RST heading, description and warning
• Split parameters into forward and trajectory sections
• Fix relative paths for literalinclude and generate_parameter_library_details

Fixes #2216

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5. New code = new tests. If you are adding new functionality, always make sure to add some tests exercising the code and serving as live documentation of your original intention.

[Bhavin-umatiya]

I have submitted a PR for this issue: #2236.
Please let me know if any changes are needed.

[christophfroehlich]

This is a duplicate of #2221, and pulling in changes from a PR which is not merged yet (there is no _from_trajectory_controller yet).