set typedoc revision to main

In theory this will stop the API docs thrashing on every build.

Of course, there is a drawback in that if main changes but we haven't
released a new version of the docs yet the links will go out of date.

So longer term we might want to investigate some kind of variable where
we can continue to have up to date revs but keep the rev in a single
variable somewhere:

https://github.com/facebook/docusaurus/issues/395

In the meantime much smaller commits will be a relief