Fix favicon URL

[laymonage]

Fixes #281

[laymonage]

Hmm looks like we can't rely on the Vercel preview, as the favicon works correctly there, even with the current main branch e.g.: https://sphinx-wagtail-theme.vercel.app/examples/code-blocks.html

Probably because readthedocs config for Wagtail would add the locale and version, whereas our config for this repo only has the latest version and without the locale.

[laymonage]

OK, can confirm this fixes the issue:

https://sphinx-wagtail-theme--282.org.readthedocs.build/en/282/examples/index.html

Compare to current main:

https://sphinx-wagtail-theme.readthedocs.io/en/latest/examples/index.html