Add opensearch description

[LennyPenny]

This allows one to easily add the search engine to the browser.

Here's how it looks on search.nixos.org:

Docs are here, maybe I'll get to it myself on the weekend:)

[SuperSandro2000]

for later reference

<OpenSearchDescription xmlns="http://a9.com/-/spec/opensearch/1.1/"
                       xmlns:moz="http://www.mozilla.org/2006/browser/search/">
  <ShortName>NüschtOS Search</ShortName>
  <Description>Simple and fast static-page NixOS option search</Description>
  <InputEncoding>UTF-8</InputEncoding>
  <Image width="16" height="16" type="image/x-icon">DOMAIN/favicon.ico</Image>
  <Url type="text/html" template="DOMAIN/?query={searchTerms}"/>
  <moz:SearchForm>DOMAIN</moz:SearchForm>
</OpenSearchDescription>

@MarcelCoding NüschtOS Option Search or NüschtOS Search? Probably the later, right? Also how do we get the DOMAIN in? We probably need to add that as a build option.

[MarcelCoding]

I would use the template string which gets replaced in the nix build.

[SuperSandro2000]

Yeah, we would need to add one for this.