Stubgen command line options reference anchors

[hoefling]

This is the follow-up of #7784. Added :option: directives in stubgen.rst.

Note: since there are two programs listing options (mypy and stubgen), a proper reference should be

:option:`program-name --option-name`

for example

:option:`mypy --py2`

(since stubgen also offers this option, so the reference to the option alone would be ambiguous otherwise).

Of course, custom text is supported, as with all roles:

:option:`--py2 <mypy --py2>`

the program name can be thus omitted in link text.

[JukkaL]

Excellent, thanks!