docs: Update mkdocs edit on github to work with addons

[cwhite911]

Enables the material mkdocs edit and view on GitHub buttons to work with docs form grass-addons.

Address part of issue: #5158

To test you need to simulate what building the addons like PR #5152 .

1. Make sure grass is built with the new md files.

2. run /grass-addons/utils/cronjobs_osgeo_lxd/compile_addons_git.sh #PR cronjobs: Add Markdown and generalize addon build grass-addons#1330

./grass-addons/utils/cronjobs_osgeo_lxd/compile_addons_git.sh 8 5 $(pwd)/grass-addons/src $(pwd)/grass $(pwd)/grass-addons/addons-build-dir grass

3. Copy the build addons docs to the dist directory of grass core.

cp grass-addons/addons-build-dir/docs/md/source/* grass/dist.x86_64-pc-linux-gnu/docs/mkdocs/source/addon

4. In grass core cd into the man directory and run the mkdocs server

cd man && make serve-mkdocs