Add `download.html` to RtD visual diff ignored files

[StanFromIreland]

I propose ignoring download.html, as currently due to the "Last updated" line the file shows up in the automated RtD comment on every PR in CPython:

This can only be done via the online RtD UI, per the documentation.

[nedbat]

I like this idea.

[hugovk]

I've added download.html at https://app.readthedocs.org/dashboard/cpython-previews/addons/edit/

[StanFromIreland]

Thanks Hugo!

[hugovk]

Shall we also ignore files like these?

• contents.html
• genindex-C.html
• genindex-E.html
• genindex-P.html
• genindex-R.html
• genindex-all.html

python/cpython#149347 (comment)

[StanFromIreland]

They are also noise, +1.

[hugovk]

Okay, now ignoring:

contents.html
download.html
genindex-*.html