Documentation

[jimschubert]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

This creates a static documentation site using Git Playbook. I'm opening the PR to get documentation feedback from the community.

To evaluate:

cd .documentation
yarn run dev

The static site generator copies files from ./docs under the repository root into the .documentation/static/docs directory for dev/prod builds. It then renders Markdown client-side. This allows the static site to give our documentation a little more structure than is available from GitHub, while keeping those docs in place with very little changes.

[jimschubert]

There are some merge conflicts, but I'll resolve them later.

For this PR, I'd still need to transform content from markdown under ./docs with relative links to markdown so they link to relative routes in the static site.

There may be other stylistic modifications as well.

[jimschubert]

Closing in favor of #1770