Universal docs testing for PRs #670
Description
Right now every team at Elastic uses its own process for testing that their docs changes are good. These processes are unique and wonderful but they all tend to leave something out. For example, the Elastsicsearch team builds the Elasticsearch reference as part of every PR. But there are a dozen "books" that the Elasticsearch repository is involved in and it doesn't build those. This lets things slip through the cracks from time to time. The Elasticsearch team uses doc_build_aliases.sh which is nice because it gives the docs team a central place to control the commands used to build the docs. Other teams do not do this so they are likely to "drift" from the canonical way to build docs. There are many repos involved with docs that don't have PR testing at all because it is time consuming to maintain the list of all of the docs to build.
I've started building (#659) some infrastructure for testing all of the books for any repository. We should be able to build on this to test docs changes in any PR. My plan at the moment is to add PR testing for some repositories that don't already have it once we have #659 and see how those shake out for a few weeks. From there I think we could shift a real team to PR testing.