[ENHANCEMENT] Add a search function to the book

[looeee]

The book needs a search function! This should be added to the left many via a search box near the "Table of Contents" heading. Search results can either go alongside, replace, or be added into the table of contents.

Lunr.js is probably the easiest to set up.

Some good info to get started here: https://gohugo.io/tools/search/