Add docs for new pip debug command

[cjerdonek]

This is a follow-up issue to PR #6638 to add docs for the new pip debug command. As @xavfernandez said in this comment:

It would also need basic documentation (at least a docs/html/reference/pip_debug.rst) and most importantly (IMHO), strongly emphasize that the output and the options of this command are provisional and might change without notice.

[pradyunsg]

I think we should print a warning on every run of pip debug -- something along the lines of:

NOTE: This command is only meant for debugging. Do not use this with automation for parsing and getting these details, since the output and options of this command may change without notice.