Update favicon

[eric-schneider]

The UI currently gets its favicon by referencing www.datastax.com/favicon.ico. See head-meta.hbs:

<link rel="icon" href="https://www.datastax.com/favicon.ico">

The benefit of this approach, in theory, is that we would always display the same favicon as WWW. However, Marketing recently updated the favicon to a .png file, which resulted in the favicon 404'ing on all of our docs pages.

Marketing temporarily restored the previous file for us, but going forward we need to make a decision: Update the URL to use favicon.png, or start self-hosting the favicon in our own UI.

I think it's highly unlikely that the WWW favicon URL will change again unless/until web standards start favoring a different file format (.ico files were in use for over two decades). However, if we want to take an offline-first approach for the sake of durability and archivability, then it probably makes sense to start self-hosting.

If we go the self-hosted route, the Antora Default UI provides the following example (modified for PNG):

<link rel="icon" href="{{{uiRootPath}}}/img/favicon.png" type="image/png">

Since our site is published to docs.datastax.com/en, the above example would put the favicon at docs.datastax.com/en/_/img/favicon.png. I assume this is fine, but we'd want to confirm that it's ok not to have our favicon at the site root.