Skeleton for a developer manual

Add a skeleton for a developer manual, mostly focusing on the used tools
for now.  Some existing content from Doxygen is linked from the new
pages, but subsequent changes will add more content (with initial focus
on explaining the formatting and documentation checking that Jenkins
does) and reformat the existing content to better fit in the new guide.

Additional sections can be added when a need is identified.

Change-Id: Id28daa9effe93fb66d03ef7b696b14ff2554e602