Add keyboard navigation to search

[xeniape]

Part of stackabletech/documentation#616

This PR adds the following navigation shortcuts to the search:

• When focus on search input field and Enter is pressed -> Focus first search result
• Navigation through search results with Up and Down arrow keys when search result in focus (the focus is basically on the link within the search result)
• Default implementation of link handling with keyboard shortcuts in browser already present
  • Press Enter to open link in same tab
  • Press Shift + Enter to open link in a new window
  • Press Strg + Enter to open link in a new tab of the same window
• Press / to get the search input field into focus again

Not an Javascript expert, so would be nice if someone could test the implementation and give some feedback if needed. Wrote a short Nuclino article on how I developed locally and how it can be tested: https://app.nuclino.com/Stackable/Engineering/Developing-with-Search-locally-bb51c5c1-d0e5-478f-8c3c-d6d64fd8d895

[lfrancke]

Thank you for the docs. I tried it locally and it worked.
For the record: I saw it fail a few times but could not reproduce. I looked at the source code as well and don't think it should make anything worse. Therefore I believe this is good to go even though it could be improved later.

Two things I'd love to improve at some point (I talked to Xenia about this)

• Pressing up/down in the search box now moves to the beginning/end of the edit box and I'd love it if it could instead move to the first/last serch result
• Only the "link" is highlighted in the search results. It'd be nice if we could highlight the entire "cell"