Publish the MkDocs SVG rendering plugin as a PyPI package

[aabounegm]

Currently, the generate_svgs.py plugin is registered as a hook. To be able to reuse it with other projects that depend on rzk, it should be packaged as a proper MkDocs plugin and published on PyPI so that others can install and use it.

Possible configuration options:

• Path to rzk binary (defaults to using PATH)

Question: Is it desirable to automatically install rzk (from GitHub releases) if not found?

[fizruk]

I think for the PyPI package by default you should assume that rzk is in PATH.