Add docs: CLUSTER SLOTS, COMMAND

COMMAND docs include individual pages for:
  - COMMAND COUNT
  - COMMAND GETKEYS
  - COMMAND INFO

If you prefer an individual commit for `CLUSTER SLOTS` vs. `COMMAND`, let me know and I'll split it out.

Also: during testing for the on-site REPL, I discovered the getKeys function for SORT is still broken.  :-\   I'm looking into why it doesn't capture the `STORE` argument.  It used to work, but then we fixed it to un-work.
Good point!  I didn't even notice—it was just an in-place copy/paste  :)

Fixed.