v3 is the default version

As [previously announced](https://developer.github.com/changes/2014-01-07-upcoming-change-to-default-media-type/), starting today, the API is now serving the v3 media type by default. This pull request updates the docs to reflect that fact.

- [x] Update all mentions of the "default" version to communicate that v3 is now the default.
- [x] Update the example responses to show the v3 representation instead of the beta representation.
- [x] Remove the inline notes on individual endpoints that explained how the v3 representation differs from the default representation. (Note that we still provide a [consolidated list of differences between beta and v3](https://developer.github.com/v3/versions/#differences-from-beta-version)).