add github repo link in headers

[samuelweiler]

Similar to what you've done for the Core spec (which also has a Github link for this doc??), add a link to this repo to the MathML 4 doc header, please?

[davidcarlisle]

The respec config has been tweaked over several commits and it is now adding several links to this github repository in the details section of the document head section (the top level repo, the commit log, the issues list)

https://w3c.github.io/mathml/