This is a good start, but more could be done to make it even more useful for
developers.
* Updated nav_template to include contributing
* Removed contributing from intro
* Fixed RST compiler error in hedits
* Renamed "OpenSubdiv" header in nav to "OpenSubdiv User Guide"
Hierarchical Edits have been deprecated in OSD 3.0, so
remove the documentation for them from the main pages.
I put the extracted docs in a file rather than deleting
them in case we want to refer to them later easily.
