Automatically updated CLI documentation on Readthedocs

[linglp]

Related to #1038

@milen-sage the workflow that Tom has (see here) could only publish documentation to github pages. But I noticed that readthedocs could let you publish documentation to readthedocs if you manually import the package and configure the webhook (without linking the project to your own github account).

More relevant documentation could be found here:

• About using readthedocs.yml: https://docs.readthedocs.io/en/stable/config-file/v2.html
• About having readthedocs to use poetry for installing dependencies: Support Poetry readthedocs/readthedocs.org#4912
• About importing project to readthedocs without linking account (checkout the second point): https://sphinx-rtd-tutorial.readthedocs.io/en/latest/read-the-docs.html#:~:text=master%24%20git%20push-,Importing%20into%20ReadTheDocs,-Once%20you%20have
• About manually configure webhook on GitHub: https://docs.readthedocs.io/en/latest/integrations.html#:~:text=GitLab%2C%20or%20Bitbucket.-,Integration%20Creation,-%C2%B6

[linglp]

For testing purpose, currently the docs would get automatically updated whenever there's a push request to develop-docs branch. But we could change the default branch on readthedocs (see Advanced setting here: https://readthedocs.org/dashboard/schematicpy/advanced/) to look at develop branch once develop-docs branch gets merged.

The documentation could be found here: https://schematicpy.readthedocs.io/en/latest/