x/website: Make website search case-insensitive

[mleonhard]

What did you do?

My golang program needs to find out if a path refers to an existing directory. I needed to find out how to do that. Here's what I did:

1. Go to https://golang.org/
2. Look around for a search button.
3. Click the three-bars icon to display the menu.
4. Click in the Search box.
5. Type isdir and hit ENTER.
6. Looked for the appropriate function.

What did you expect to see?

A link to FileInfo.IsDir() bool

What did you see instead?

Irrelevent links. Here's a screenshot:

[rsc]

Search box removed (#44357).