[www] Open search form with keyboard shortcut

[m-allanson]

Add keyboard shortcut / to open the search form on gatsbyjs.org.

Ref: #3421 (comment)

[markmichon]

Is this convention? Some browsers default / to Find. (like Firefox)

Currently, it's the first tab target, which generally a good a11y practice.

[KyleAMathews]

Oh hmmm good point. I've done it on a web app before and it was great but perhaps it's not worth doing it here.

[m-allanson]

Maybe we could look at how GitHub handles it? On the issues page / opens Firefox's built in 'Find in page' control, but for other browsers it focusses the issue search box.

[m-allanson]

We could also use a key that doesn't conflict with any existing browser shortcuts. Check out the full range of GitHub shortcuts with ?.

[KyleAMathews]

Yeah, they use s which seems reasonable. We could also use copy their "go to" commands e.g. gd is go to docs, gp is go to package search page (once that's built that is :-)). Which would be fun too.