Create a keyboard shortcut to focus the search input

[colebemis]

@brianlovin mentioned that searching for octicons is probably the first thing that people want to do when they go to https://primer.style/octicons. Let's add a keyboard shortcut (probably /) to focus the search input.

Notes

We currently have two search inputs on the https://primer.style/octicons page:

This is a problem we should fix in a separate issue. In this issue, let's focus on the search input that filters the grid of icons.

[github-actions]

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