doc: replace OPEA_API word doc with rst version (#94)

* The word (.docx) version of the OPEA API specification was stored in
  the docs repo and wasn't convenient for developers to read or
  maintain.  Convert the word doc to reST. Note though this is a
  conversion that maintains some technical errors found during the
  transcription.  The intention is to make this API spec more visible
  and able to be reviewed and updated easily in this reST form.

* Create a more easily found document structure for this and other
  developer guides (previously in the community area) and move a few
  around to fit into this new navigation structure.

Signed-off-by: David B. Kinder <[email protected]>
Signed-off-by: Tom Lenth <[email protected]>