[New docs] Setup local search

[tkajtoch]

Summary

As part of #7753 we want the new documentation site to be searchable by page titles, content, component names, and more. To start this effort, we should setup a local search docusaurus plugin that uses lunr.js as its search backend for simplicity. This way we can have version-aware search that doesn't require server-side infrastructure.

Acceptance criteria

• docusaurus-lunr-search should be set up and working
• A search bar should be added to the top nav bar
• Pages should be searchable by their titles and headings