add show_code_download conf.py option

[LuisBL]

Related to #1069

provide a show_code_download key to conf.py, if False do not expose "Download Python source code" neither Jupyter notebook

[larsoner]

See #949, I think we've decided this is not necessary

[SimenZhor]

@larsoner as a user I think it would be a lot more convenient to have this as an option in conf.py rather than having to make custom css rules to hide the buttons. Especially as that would eliminate the need for uploading the _downloads folder to my host (this PR should probably make sure that folder does not be generated in the first place).

[larsoner]

@lucyleeow WDYT?

[lucyleeow]

I understand it would be easier to users but I am reluctant to add to our already large list of config options (though it probably doesn't add much maintenance).
How about adding a part to our documentation about how to hide this via CSS? Maybe expanding the section 'And some things can be tweaked directly in CSS:' (here). I vaguely recall a few issues about other things that can be tweaked via CSS.