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