refactor: Migrate Handbook to Sphinx Static Site Generator

[kgpayne]

Given the companies first language is python 🐍 and the use of Sphinx for other site in the Meltano estate (e.g. the SDK) it would be straight forward to migrate the existing handbook content and layout to Sphinx. The benefits of a switch include:

• One less static site framework to learn (reduced learning curve for maintainers)
• Direct reusability of assets across sites (consistency in look and feel)
• Close parity of experience for content contributors (still just markdown in a directory tree)

[kgpayne]

In particular, this would more easy accommodate additional levels of hierarchy in the nav (which in Jekyll is a template html change).