Docs: Explain what goes into the redcarpet: extensions: array

- Document the two secret-sauce extensions that Jekyll adds, which don't seem to
  be mentioned elsewhere.
- Link to the Redcarpet documentation for other extensions.
- Mention the three or so most common extensions.
- Mention that Redcarpet 2.2.2 lacks several extensions that were added later.
Merge pull request #1418 from nfagerlund/docs_redcarpet

Docs: Explain what goes into the redcarpet: extensions: array