start doc'ing py goals. set up python_old for old UI, start refactoring python-readme.

Python supports 'goal test' and 'goal binary' now. python-readme should reflect that.
This change doesn't 100% fix that; but it's a step in the right direction.

Twitter's biggest python-Pants codebase is on an old Pants.
(also, a couple of open-source Pants-built repos)
This makes me hesitate to blow away the old doc just yet.

This change...

  copies python-readme to python_old.

  adds paragraph at top of python-readme "Are your Pants old? Then see python_old"

  rm'd some historical python-readme parts now less-obsoletely doc'd on other pages

  mv'd some PEX-design-background material onto a new page pex_design.
  (It might make more sense for me to mv this to twitter/commons?
   I dunno. If you tell me to move it to twitter/commons, I will.)

  mv'd some python-readme parts to other pages

  Added a couple of TODO github issue links for biggish problems noticed
  along the way.

I'm not trying to get python-readme perfect in this pass.
This is mostly moving material around.

That said, a couple of examples now use 'goal'. YAY

Testing Done:
./build-support/bin/publish_docs.sh # stuff looks OK

./build-support/bin/ci.sh # docstring changes apparently nonfatal

Bugs closed: 258

Reviewed at https://rbcommons.com/s/twitter/r/594/