Add search functionality

[suimong]

Is your feature request related to a problem? Please describe.
Currently the doc site lacks full text search functionality which could greatly improve user experience.

Describe the solution you'd like

A frontend-only solution appears to be more suitable. lunr.js seems popular (e.g. mkdocs for material integrates it), although maintenance is somewhat lacking. Algolia docsearch is another popular choice (integrated by docusarus), and could be even easier to setup as they provide free cloud API for developer docs.