docs: Added material mkdocs tags plugin

[cwhite911]

Addresses Issue #5091

This pull request includes multiple changes to improve the documentation and tagging system in the project.

Modified md keyword generation in documentation:

• lib/gis/parser_rest_md.c: Changed the keyword output to use "tags" instead, and adjusted the format to include brackets.
• lib/gis/parser_rest_md.c: Removed the conditional keyword section to streamline the output format.

Updates to build process and documentation files:

• man/Makefile: Added tags.md to the list of documentation source files and ensured it gets installed correctly. [1] [2]
• man/mkdocs/mkdocs.yml: Enabled the tags plugin in the MkDocs configuration to support tag-based navigation.
• man/mkdocs/overrides/partials/footer.html: Updated the footer to include a link to the new Tags index.
• man/mkdocs/tags.md: Added a new file to list relevant tags for the documentation.

[wenzeslaus]

While just using the tags instead of a currently working custom keywords system is not a priority, and some pieces will need to me preserved for keeping functioning URLs (if someone links https://grass.osgeo.org/grass-stable/manuals/keywords.html), this may help solving keywords for addons which currently work for HTML, but not for Markdown. However, given this needs the addon situation to be resolved, let's wait with this a bit to see how #5152 develops.

Example with a addon-only keyword is v.vect.stats.multi.md with keyword 'columns'.

[cwhite911]

While just using the tags instead of a currently working custom keywords system is not a priority, and some pieces will need to me preserved for keeping functioning URLs (if someone links https://grass.osgeo.org/grass-stable/manuals/keywords.html), this may help solving keywords for addons which currently work for HTML, but not for Markdown.

This is an easy fix. We just need to leave the keyword tag in the header for now.

Example with a addon-only keyword is v.vect.stats.multi.md with keyword 'columns'.

I'll need to see this because I'm not following.

However, given this needs the addon situation to be resolved, let's wait with this a bit to see how #5152 develops.

I agree this can wait until we resolve the addons.