Don't advertise legacy watch* endpoints

The watch* endpoints are available in the github.beta media type *and*
the github.v3 media type. But, let's not advertise them. People should use
the star* endpoints instead.

See blog post:

http://developer.github.com/changes/2012-9-5-watcher-api/#update---november-6-2013
Introduce prototype for beta-to-v3 changelog
Fix whitespace
Move alert into "Response" section
Document deprecation of `master_branch` attribute

- Add `default_branch` attribute to example responses
- Document deprecation of `master_branch` attribute
Fix whitespace
Deprecate `public_gists` & `private_gists` attributes
Fix whitespace
Deprecate beta /user/emails response format
Consolidate version info on new "Versions" page

- Create new "Versions" page
- Move v3 "Deprecations" section to the new page
- Move "Breaking Beta Changes" section to the new page
- Move "Beta-to-v3 Changelog" section to the new page
- Link to the new page from the "Current Version" section of the API
  overview page
- Link to the new page from the "Media Type" page
Fix links
Spread the word about v3
Merge remote-tracking branch 'origin/master' into clarify-beta-v3-differences
Add 'Versions' page to search index