Serve plugin docs from their dedicated repos

[reynoldsalec]

As of Lando 3.6.0 all integration partner plugins (Pantheon/P.sh/Acquia/Lagoon) are being served from individual repos. They also include their documentation in those separate repos, but for that 3.6.0 release we had to manually copy over the doc files into this lando/docs repo.

In this task we should automate the generation of the plugin docs from their dedicated repos and have them integrated into our docs site as expected.