For 7.x and earlier branches, `_cat/repositories` API requests require a
repository name.
This removes an erroneous request example without a repository name
added with a8e0275.
Several files in the REST APIs nav section are included using
:leveloffset: tags. This increments headings (h2 -> h3, h3 -> h4, etc.)
in those files and removes the :leveloffset: tags.
Other supporting changes:
* Alphabetizes top-level REST API nav items.
* Change 'indices APIs' heading to 'index APIs.'
* Changes 'Snapshot lifecycle management' heading to sentence case.
James Rodewig
Nik Everett
Yannick Welsch