Mark the legacy search API as deprecated

- Add deprecation notice at the top of the legacy search API docs
- Update "Expected Changes" to announce plan to remove the legacy search API
  in the next major version of the API
Remove mentions of "preview" mode in Search API docs

- Remove the "preview mode" section
- Update all the Accept header examples to specify "v3" as the API version
  (instead of "preview")
Remove mentions of "preview" mode in Rate Limit API docs
Mark `rate` hash as deprecated in Rate Limit API
Remove trailing whitespace
Recommend the v3 media type (instead of beta)
ANN: Search API Becomes an Official Part of API v3