Removes `favicon.html`, shifts content to `head_custom.html` (#1027)

* Removes `favicon.html`, shifts content to `head_custom.html`
* explicit callout for custom favicon in customization docs
* Cleaner and more consistent documentation (review from @pdmosses)

Co-authored-by: Peter Mosses <18308236+pdmosses@users.noreply.github.com>