docs: Generate manual from markdown

[landam]

Goal

Build user manual from Markdown using mkdocs

How to test

1. Install requirements, see man/mkdocs/requirements.txt
2. Compile GRASS
3. Open dist.x86_64-pc-linux-gnu/docs/markdown/site/index.html in web browser

Tasks

• fix gui/script/ compilation
• fix code blocks
• fix commit link
• implement build_md.py and indices
• add missing meta page comments to MD files (how to do it with pandoc?)
• remove toc-related code from mkmarkdown.py and change mkdocs style to show page-related TOC
• fix "## DESCRIPTION" appearance from keyword list
• fix CI
• separate makefile rules (Markdown.make, MarkdownRules.make) ?
• make install
• logo points to index.html
• site_name to "GRASS GIS 8.5.0dev Reference Manual"
• fix menu items

Tasks before merge

• rebase main and regenerate MD files from HTML once again

Tasks after merge(?)

• remove rest-related scripts
• generate man from markdown
• remove HTML-based documentation and clean-up scripts

Notes

How HTML was converted to MD:

for f in `find . -name *.html`; do echo $f; pandoc -i $f -o ${f%%.html}.md;done