Add doc on C++ API

[Abishalini]

This is an attempt to add more information on stages provided by MTC and how to use them using C++.

How to build the docs

cd ~/path/to/moveit_task_constructor
sphinx-build core/doc _site

The HTML files can be accessed in the _site folder.

Make sure you have the latest version of Sphinx by pip install -U sphinx.
Since I primarily work with ros2, I don't have some dependencies required to build the docs. You might encounter this error

Extension error (sphinx.ext.autosummary):
Handler <function process_generate_options at 0x7fc729afc8b0> for event 'builder-inited' threw an exception (exception: no module named moveit)

If you encounter the error, comment out the Python section from api.rst

TODO:

• Potentially reorder sections. There is some duplicate information between the Python examples and the C++ examples I have added
• Figure out versioning between master and ros2 branches.
• Add more information/pictures about debugging using the RViz.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c605a00) 59.02% compared to head (3247989) 58.80%.
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #511      +/-   ##
==========================================
- Coverage   59.02%   58.80%   -0.22%     
==========================================
  Files          90       90              
  Lines        8482     8705     +223     
==========================================
+ Hits         5006     5118     +112     
- Misses       3476     3587     +111     

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