add favicon to docs

[kaczmarj]

see https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-html_favicon

we will have to

• make an ico file (i think 32x32 pixels) using our logo https://wsinfer.readthedocs.io/en/latest/_static/logo.svg
• save that to docs/static/favicon.ico
• set the html_favicon variable in conf.py for sphinx