Add doc validation on merge

[ifropc]

What problem does your feature solve?

We should add documentation validation on merge. Currently we might accidentally push a change that breaks stellar-docs but we can't know that

What would you like to see?

GHA for validation.
We can clone stellar-docs and do the build on latest change (this would require change to the stellar-docs to allow to do that though)

What alternatives are there?

N/A

[ifropc]

We should also make docs to pull docs artifact from the latest release