Create Azure function for Building Roslyn Analyzer documentation

From @mavasani (https://github.com/dotnet/roslyn-analyzers/pull/2671#discussion_r303506897):

Context: We have some analyzer documentation files that are checked into the repo, and these get auto-updated when cibuild.cmd is executed locally. However, it is not reasonable to expect every dev or PR to run this locally prior to submitting a PR. Our current model is that I periodically execute this locally on a new branch and submit a PR, but it would be good if we can setup automated PRs (similar to merge PRs across branches) where the PR just does the following:

1. Fetch the latest master branch sources and git checkout a new local branch.
2. Execute eng\common\cibuild locally
3. If there are any diffs in the source tree, submit a PR to roslyn-analyzers repo with the change. Otherwise, bail out without submitting a PR.

Having the above automated PR generation execute at the same frequency as automated branch merge PRs would suffice.

An example PR is here: [#2676](https://github.com/dotnet/roslyn-analyzers/pull/2676). I just executed eng\common\cibuild on latest master sources, pushed a commit on a new branch and submitted a PR.

Start by reviewing the automated branch-merge PR workflow referenced in the issue and the example PR #2676. Run eng\common\cibuild against the latest master sources to observe the generated documentation diffs. Done means an Azure-based automation fetches master, creates a branch, runs the build, and opens a PR only when changes exist.

Written by the indexing model from the issue text.