Reorganizing docs as recommended in:

https://www.divio.com/blog/documentation/

This is simply a reorganization of the existing documents and changes
no content EXCEPT to correct the location of sphinx doc references.
Expect followup changes to change document names (to reflect the new
structure) and to move content from existing guides (e.g., to move the
pipeline/project/job structure definitions out of the "Project Configuration"
reference guide into their own reference documents for easier locatability).

All documents are now located in either the "overview", "tutorials",
"discussions", or "references" subdirectories to reflect the new structure
presented to the user. Code examples and images are moved to "examples" and
"images" root-level directories.

Developer specific documents are located in the "references/developer"
directory.

Change-Id: I538ffd7409941c53bf42fe64b7acbc146023c1e3