Expose a lot of "tech docs" via "git help" interface.
* ab/tech-docs-to-help:
docs: move http-protocol docs to man section 5
docs: move cruft pack docs to gitformat-pack
docs: move pack format docs to man section 5
docs: move signature docs to man section 5
docs: move index format docs to man section 5
docs: move protocol-related docs to man section 5
docs: move commit-graph format docs to man section 5
git docs: add a category for file formats, protocols and interfaces
git docs: add a category for user-facing file, repo and command UX
git help doc: use "<doc>" instead of "<guide>"
help.c: remove common category behavior from drop_prefix() behavior
help.c: refactor drop_prefix() to use a "switch" statement"
c0f6dd49f1