Document & announce the new Search API

- Move legacy search docs to a separate page (@gjtorikian)
- Document new search API (@gjtorikian & @jasonrudolph)
- Describe new /rate_limit response format (@jasonrudolph)
  - Move Rate Limit API to its own page
  - Describe the new (coming soon!) response format
- Grab @gjtorikian's CSS from a recent PR [1] to highlight the fact
  that the Search API is in 'preview' mode (@jasonrudolph)
- Sort "Overview" subsections in TOC (@jasonrudolph)
- Fix hash linking TOC render (@gjtorikian)

[1]: https://github.com/github/developer.github.com/pull/266/files#diff-1