Add favicon

[thomasmarshall]

This PR adds a favicon which will be shown in the browser tab or when someone bookmarks the Ruby documentation site. Ideally we would add a link tag in the HTML files, but most browsers will default to /favicon.ico if the link isn't defined, so this is easier than configuring a custom RDoc template or post-processing the files.

I copied the favicon.ico file directly from ruby/www.ruby-lang.org.

Here's an example of the bookmark I keep in my browser bookmarks bar:

Before	After

Perhaps this doesn't seem important, but the reason I have a bookmark is that it's currently quite difficult to reach official Ruby documentation via Google search. For example, try searching for "ruby array join" and you will see that the official Ruby docs are half way down the second page of results. As such, it would be great if the bookmark was easier to scan for visually.

[st0012]

FWIW, I think RDoc should support customizing favicon as well. So I opened ruby/rdoc#1192

[st0012]

Has this been deployed? Checking because https://docs.ruby-lang.org/favicon.ico still returns 404 for me.

[thomasmarshall]

I saw your latest documentation style changes have been deployed @st0012 but the favicon is still 404. I don't know what the deployment process is—I assumed files in public would be publicly available at the root but perhaps that's not correct.

[hsbt]

Don't rush that. I'll look that at next week.

[hsbt]

https://docs.ruby-lang.org/favicon.ico is available now.

[st0012]

@hsbt Thank you!