Link to the auto-generated documentation in GettingStarted.md

Our GettingStarted.md developer document still links to the old torguts git repository: https://gitweb.torproject.org/tor.git/tree/doc/HACKING/GettingStarted.md#n53

But it should link to our new auto-generated documentation.

I don't know the details of the new docs website. But it would be nice to have it done for Outreachy.