The major change in this PR is that it removes completely the ability to add a warning banner statically using sphinx internals. Ref #13

There is an example of a banner with flask-style at https://sphinx-version-warning-example.readthedocs.io/en/flask-like-warning/ that uses this PR to generates it.

Also:

• makes the extension visible under versionwarning.extension, instead of under versionwarning as it was before. It avoid possible circular imports.
• removes unnecessary files.
• all the config options are included/considered in the json file.

ToDo:

• update documentation
• manual QA
• write tests to check the generated .json file

Closes #13