Create and link sitemaps for the combined MkDoc doc and Sphinx doc

[wenzeslaus]

Create sitemap for the MkDoc doc and the Sphinx part of the doc whole combined doc in CI (Documentation workflow) and prepare sitemap for the main website which reflects new names or file placement if needed.

• current sitemap structure fully mapped
• sitemap generated for main doc
• sitemap generated for addons doc (as one file with the main doc)
• sitemap generated for Python (Sphinx) doc
• sitemap for core doc and linked from main sitemap
• sitemap for addons and linked from main/parent sitemap
• sitemap for libpython and linked from main/parent sitemap
• main sitemap at grass.osgeo.org prepared for a new site map structure

Notes:

• MkDocs generates sitemap.xml (and sitemap.xml.gz) which contains both the main dir and addons subdir with https://grass.osgeo.org/grass-stable/manuals/ URLs and lastmod date.
• Sphinx has an extension sphinx-sitemap.

The sitemaps are currently created on the server by a custom script:

############################################
# create sitemaps to expand the hugo sitemap

# versioned manual:
python3 $HOME/src/grass$GMAJOR-addons/utils/create_manuals_sitemap.py --dir=/var/www/code_and_data/grass$GMAJOR$GMINOR/manuals/ --url=https://grass.osgeo.org/grass$GMAJOR$GMINOR/manuals/ -o
python3 $HOME/src/grass$GMAJOR-addons/utils/create_manuals_sitemap.py --dir=/var/www/code_and_data/grass$GMAJOR$GMINOR/manuals/addons/ --url=https://grass.osgeo.org/grass$GMAJOR$GMINOR/manuals/addons/ -o

# grass-devel manual:
python3 $HOME/src/grass$GMAJOR-addons/utils/create_manuals_sitemap.py --dir=/var/www/code_and_data/grass-devel/manuals/ --url=https://grass.osgeo.org/grass-devel/manuals/ -o
python3 $HOME/src/grass$GMAJOR-addons/utils/create_manuals_sitemap.py --dir=/var/www/code_and_data/grass-devel/manuals/addons/ --url=https://grass.osgeo.org/grass-devel/manuals/addons/ -o