Docs: Add documentation site

[kraftbj]

To provide documentation for the plugin, we're spinning up a new "handbook" on developer.wordpress.org (DevHub).

Work on the developer.wordpress.org site itself is tracked via https://meta.trac.wordpress.org/ticket/7917

The source of truth for the documentation will be the docs folder within this repo. See #31.

I made an initial pass on a documentation set, but it is very far from complete or robust. PRs accepted to add/update documentation.

Documentation for existing, released features can include a composer docs run to update the manifest file used for ingesting to DevHub. If a feature is totally new for an upcoming release, we can defer composer docs until when finalizing the release.

[kraftbj]

Current status: The DevHub site is visible at https://developer.wordpress.org/secure-custom-fields/ and not currently linked from the homepage.

The manifest is being ingested with pages created. There is automatic parsing of docblocks, converted to markdown.

The ingestion isn't right yet; some files lacking titles, the "SCF" page is being added anew over and over again, etc. Progress, though.