Change from sphinx-build and reStructuredText to mkdocs and MarkDown as per GSIP-221

Jody Garnett (geocat.net)
Krishna Lodha (rottengrapes.tech)

Pull request: https://github.com/geoserver/geoserver/pull/7429

Preview: https://jodygarnett.github.io/geoserver/

PR needs reviewing

Please use the tracking spreadsheet: https://docs.google.com/spreadsheets/d/1HMqUpTirEwwSQfeNYikPjU0aFMl0-W4Xj44cg8V_VJA/edit?usp=sharing - request edit access