Bluetooth: write documentation

Write an app_bluetooth page documenting the APIs for adapter state,
device information and discovery. Update links accordingly.

BUG=256728
R=armansito@chromium.org, mkearney@chromium.org

Review URL: https://codereview.chromium.org/189263004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256260 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256260