Add developer starting guide into the docs

[spisakt]

The documentation is sphinx-based and gets automatically render by ReadtheDocs.io.

Make a new "menu point" in Docs/index.rst, called "Developer guide".
This points to a new file, which will describe various options in the future.
Copy your pycharm+Docker sop there.

The doc pages already contains a small description about how to do it:

https://pumi.readthedocs.io/en/docs-test/dev_docs.html

Refer to the sphinx user guide for more details (e.g. how to render the docs locally for testing, how to upload images, etc.).