Add search feature to docs

[Salt-Factory]

The docs could really use a search feature to quickly find information. According to the docusaurus docs, this should be doable.

I can look into this myself as soon as I find some time.

https://v2.docusaurus.io/docs/search/

[tasn]

I'm OK with doing it, though it relies on a 3rd party provider, which is why it hasn't been done yet.

[tasn]

I think it now supports static search (no provider), happy to add that!