Avoid the need to add a link to favicon,ico when editing `_includes/head_custom.html`, and avoid creating an invalid favicon link
- Remove the content of `_includes/head_custom.html`
- Add code to `_includes/head.html` to create a link to an existing favicon,ico
- Add an explanation of favicon_ico to docs/configuration.md
- Remove the example of `includes/head_custom.html` and add an explanation of what the `<head>` element automatically includes
* 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>