Better navigation via tags, especially in book pages

**Is your feature request related to a problem? Please describe.**

[This page](https://hackmd.io/islS-k8NTOWOlzMybr5ZXw) is tagged with `Buddhism` and is part of [this book](https://hackmd.io/AIr-zgMJRnqWPEqLKolB7g). However, on that page there is no way to navigate to other similarly tagged pages (an index, if you will) nor the book it is part of.

**Describe the solution you'd like**

I'd like to have a public facing index of tags so people can more readily browse related pages. Or, additionally, the ability to navigate up to the book a page is part of -- belonging to multiple books is a possibility to consider.