[Feat] Improve the local MkDocs experience

[wenzeslaus]

This is a general issue to track tasks, bugs, and missing features in the MkDocs documentation when it is used locally.

Specifically, this is focused on the documentation is used as full HTML, built by MkDoc from Markdown and installed together with GRASS binaries, but displayed in a web browser. There is some overlap with just general offline documentation based on Markdown in general because of how GUI Help tab works and man pages are generated, so I'm including it here as well.

Basic features:

• g.manual uses MkDocs-built HTML files (if present). g.manual: use new markdown docs #5110 - This shows the full HTML file (static, not served).
• GUI shows the MkDocs-built HTML files (if present). GUI: prepare for markdown documentation #5066 - This simplifies the full HTML into something which the embedded wxPython HTML render can display.
• man pages are created from Markdown. doc: Markdown to man conversion #5358

Desired features:

• Search is functional, hidden, or clearly disabled.
• Matomo analytics is not included or fully disabled.
• User asking for an addon page results in a page being shown (online or offline).