Implements #287 with a few adjustments / proposals up for discussion.

@agnieszka-m
Search can be opened via icon in header. Input field has autofocus enabled.
Search can be closed by clicking outside the modal or hitting Esc while input field is focussed.
Search is triggered by hitting enter.

Before testing, adjustments to the .env.local are necessary - I assume that will also need to be set in Vercel.

Considerations / decisions to be made:

• Display of search results: Due to the way the documents are indexed in Haystack, the search can and will often return partial markdown. We cannot render this to HTML reliably and displaying it in its original markdown state is not very pleasant. I'd propose to change the search endpoint in a way that it returns plain text. I am unfamiliar with the inner workings of Haystack, but I assume that is something that could be possible?
• Opening the search: I could add a CMD / CTRL + K keyboard shortcut to open the search, making it easier to access. Is that something that would fit what you are going for?
• Debounced search: Instead of waiting for the user to hit enter we could add a ~250-500ms debounced auto search after the user stops typing. Is this required?
• Versions: With my examples I often got only results from older versions. The issue specifies that we should search only for the currently selected version, in my tests the result yield was somewhat low though. I have for now removed that filter and just sorted the latest version to the top, I am happy to revert that though if you prefer to stick to the filtered search.
• Dead links: I have encountered a couple of dead links in the search results, that is something we should probably check for in the indexing process.