Added docgen documentation
Fixes two minor typos.
Reformats text to fit width of 80 columns.
Adds docgen to list of documentation to build.
Adds some cross references to docgen manual.
Transforms docgen sample to be generated from source.
Merge branch 'pr_oniondocs' into pr_oniondocs_devel
Merge pull request #841 from gradha/pr_oniondocs_devel

Added docgen documentation to devel branch