Fix paths on search results

What does this MR do and why?

Makes a few fixes for search result links:

• Use a relative URL rather than an absolute URL
  • This should allow links to resolve on subdirectory-style instances. We can do this in our JS code by using docsBaseURL() rather than ever linking to something with a leading /.
• Adds a _redirects file to try rewriting Nanoc-style URLs to Hugo URLs (#88 (closed)). If these redirects work, result links should actually resolve (after a redirect) on the Hugo site 🤞

How to set up and validate locally

Can't really test this locally (especially the redirects).

Merge request acceptance checklist

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• I have evaluated the MR acceptance checklist for this merge request.