Add some JS code to update GitHub stats for repos in docs

[wragge]

I've updated the mkdocs-material theme. The theme expects there'll be one repository, but I've got lots. I'd previously hacked the templates to embed repo details on individual pages. But with the update, the stats only seem to be downloaded once then cached. So the cached results get displayed on every page.

I should be able to add a new javascript function that gets and displays the individual repo stats.

[wragge]

I decided that there was no reason for JS, as the numbers don't change that often, so I put the code into a mkdocs plugin.