docs: General todos for Markdown

[wenzeslaus]

This is a general todo list for the new documentation which originally lived in #3849 (I transferred the unresolved todos only). Feel free to add more basic issues here, but use smaller issues and the project for specific tasks if possible rather than turning this into a un-closeable mega issue.

Needed

• add missing meta page comments to Markdown files (as YAML metadata block) docs: Use meta page info from HTML in Markdown #5169
• keep supporting HTML-only in g.extension for the addons
• parser_standard_options: use formatting other than a table docs: Replace table by sections for standard parser options #5235
• main index: choose better formatting than the current table docs: mkdocs index and nav redesign #5207
• add GRASS_PROXY to Markdown build (not ported from HTML to Markdown in docs: Generate manual from markdown using mkdocs #3849; variable introduced in mkhtml: Get date from git log #2100) docs: Add proxy settings to Markdown build #5236
• generate man from Markdown (and not from HTML)

Notes on man:

-$(MANDIR)/%.$(MANSECT): $(HTMLDIR)/%.html
+$(MANDIR)/%.$(MANSECT): $(MDDIR)/source/%.md

Nice to have

• manual_gallery: different number of images in Markdown vs HTML (run in man dir: GISBASE="/home/martin/src/grass/dist.x86_64-pc-linux-gnu" ARCH="x86_64-pc-linux-gnu" ARCH_DISTDIR="/home/martin/src/grass/dist.x86_64-pc-linux-gnu" VERSION_NUMBER=8.5.0dev VERSION_DATE=2024 python3 ./build_manual_gallery.py) and see dist.x86_64-pc-linux-gnu/docs/markdown/site/manual_gallery.html and https://grass.osgeo.org/grass84/manuals/manual_gallery.html build: Fix basic gallery build issues for Markdown #5247
• manual_gallery: fix layout (now large images without descriptions comparing to thumbnails with descriptions) docs: Add captions to Markdown manual gallery #5250

Future

• revise usage of keywords and tags from user perspective (what should a reader see)
• revise usage of keywords and tags from implementation perspective (valid YAML syntax and duplication)
• revise or clean up the graphical index (index pages) - drop it or make it the only index pages
• remove reStructuredText-related code and scripts (aka rest or rst)
• remove HTML documentation files
• remove custom HTML documentation from build system and build scripts

[cwhite911]

main index: choose better formatting than the current table

I have an idea for this one, but it will be easier just to show in a draft PR.