Makes docs generation work on my computer (basically)

[localhostdotdev]

• --verbose, --target and --tags flags (--help is provided for free by optparse)
• Changes from all the other PRs combined into one PR basically
• [x] rake test pass
• [x] rake test:all pass because doc_generator_test.rb is in "Pending." state :)

So this is mostly to give a higher level view of all the changes and it quite nicely lets me do:

bin/generate_docs.rb -t checkout/ -v --tags=v5.2.0

And then python3 -m http.server lets me see the generated docs.

The main thing is I'm waiting for some high-level approval on this PR before writing tests for the three new flags. Maybe trying to revive the old tests too.

(Obviously I just want to fix the links thing on the generated docs and it's related to rdoc but might be much more complicated to have something merged in rdoc than here)

(And this PR could use a good rebase too)