docs.rtems.org needs changes to reflect gitlab and master->main rename

Summary

https://docs.rtems.org/ still has /master/ as the URL. This needs to be changed to main we can have an automatic forward in the webserver to redirect https://docs.rtems.org/*/master/* to `https://docs.rtems.org/*/main/*`.

This step needs to be completed before docs can be automated anyway so it is a blocking change to that but I'm unsure if CI doc building will happen before %6.1 or not.