doc: Changed go/ category etc. to topic/

Per instructions from DD and JF, changing the top-level go/
directory to topic/ ; as a result, I'm going into AK's recent CLs
(in which she added infrastructure for the new go/ directory)
and changing those too.

See first comment for doc stage location.

bug: 28558659
Change-Id: Iaa10f1a9f7477b00f6bfaa748039d146dfb292fe