Write documentation on Trestle Plan, Action and Element

[leninmehedy]

We need to document the concept, architecture of Trestle Plan and Action. The documentation should also include guide and example on creating a Trestle Plan. The document should reside in the /docs folder for now.

[butler54]

This should all be done in docstrings in the classes and referred to in the markdown documentations.

[vikas-agarwal76]

Enhance the documentation in actual commands.