Make docs contribution more flexible

[cnunciato]

Today, we expect package authors to craft Hugo-specific Markdown files, named in a very specific way and containing specific frontmatter fields, to comprise their packages documentation.

This is both too restrictive (authors only get two docs -- one overview page and one install page) and too implementation-specific (authors shouldn't have to deal with the esoteric details of Hugo or whatever other platform we're using in order to ship their packages, nor should we expect them to come up with page titles, meta descriptions, etc., for us).

Instead, we should allow authors to submit plain GitHub-flavored Markdown files for their overview and install docs. Moreover, we should also provide some way for authors to provide additional free-form pages (e.g., how-to guides, upgrade/migration guides) to help their users address common scenarios.