Add favicon to Read the Docs

[hugovk]

Re-uses the favicon from https://github.com/python-pillow/python-pillow.github.io/blob/master/images/pillow.ico

• Before: https://pillow.readthedocs.io/en/stable/
• After: https://pillow-hugovk.readthedocs.io/en/add-docs-favicon/

Edit: on a Retina screen:

[radarhere]

The documentation says that the ico file should be 32 pixels at most, but copying what we're already doing on another site seems perfectly reasonable.

[hugovk]

Yep, saw that, and seems to work. The ico file does also contains a 32x32:

$ identify docs/resources/favicon.ico
docs/resources/favicon.ico[0] PNG 256x256 256x256+0+0 8-bit sRGB 102602B 0.000u 0:00.009
docs/resources/favicon.ico[1] ICO 48x48 48x48+0+0 8-bit sRGB 102602B 0.000u 0:00.000
docs/resources/favicon.ico[2] ICO 32x32 32x32+0+0 8-bit sRGB 102602B 0.000u 0:00.000
docs/resources/favicon.ico[3] ICO 24x24 24x24+0+0 8-bit sRGB 102602B 0.000u 0:00.000
docs/resources/favicon.ico[4] ICO 16x16 16x16+0+0 8-bit sRGB 102602B 0.000u 0:00.000

[hugovk]

Now on https://pillow.readthedocs.io/en/latest/

And next release at https://pillow.readthedocs.io/en/stable/