Extend asciidoc GitHub Action workflow to manage publication of docs

[lesserwhirls]

In addition to building the conventions and conformance docs, extend the GitHub Action workflow to manage the publication of the docs to the gh-pages branch based off the workflow of the travis.yml config.

Demonstration

Pull request against master

PR: https://github.com/lesserwhirls/cf-conventions/pull/9
Action: Conventions and conformance documentation built, preview artifact uploaded
No changes to gh-pages branch.

Pull request merged into master

Action: Conventions and conformance documentation built, documentation updated in root directory of gh-pages branch.
After Action: gh-pages branch.

GitHub release published

Action: Conventions and conformance documentation built, documentation updated in release version directory of gh-pages branch
After Action: gh-pages branch.

Closes #297

[lesserwhirls]

I just realized forgot to remove the workflow_dispatch event trigger (thus the update), but now the PR should be good for review.

[sadielbartholomew]

The new workflow looks sound to me and it works based on the Actions runs and logs. Great stuff.

[erget]

Thanks @lesserwhirls and @sadielbartholomew , merging!