Need to generate review-able docs from PRs in submodules.

[wmamills]

Any doc changes in the sub-modules should be reviewed before they get to the submodule's main branch.
That is possible for people to do on the desktop but not easy or convienyant so it will likly get skipped.

We should generate readthedocs "PR" content for such changes and put the link into the submodule's PR.

How to do this is TBD for now.

[wmamills]

This is in progress. See OpenAMP/libmetal#256

[wmamills]

This now works for libmetal, open-amp, and openamp-system-reference.

@nathalie-ckc we need to ask Bruce if he wants this for lopper. I don't have permissions in that repo to enable the hook.