Text search within a book (not just titles)

**Is your feature request related to a problem? Please describe.**
Choose your favourite hackmd book. E.g. [this one](https://hackmd.io/c/tutorials/%2Fs%2Ftutorials). You can only search **titles** within the book, and not the rest of the book. Books would be far more useful if you could search their entire contents (e.g. paragraphs etc.).

**Describe the solution you'd like**
The ability to search _everything_ within a hackmd book; not just the titles of notes and headings. The functionality to search _everything_ already exists as a prime member, from the home page's search. But that functionality hasn't made its way into a book, unfortunately.

**Describe alternatives you've considered**
I've considered Gitbook, GitHub Pages (via MDBook), Obsidian, etc. All of these have better search functionality within a book.

**Additional context**
None