Add breadcrumb links to parent sections in the book theme

[choldgraf]

For multi-page projects (ie, those that would use the book theme), you often have content that's nested inside of other content. In this case, it's useful to have some context about the "parent" of a page and where it sits in the project hierarchy. This is often called breadcrumbs in websites.

I think we should allow book authors to include breadcrumbs in their site, it would have a structure like 🏠 > [parent page link] > current page, maybe somewhere around here:

And triggered somewhere around here:

project:
  site:
    options:
      breadcrumbs:
        display: true # true by default

Examples

Here's an example from pydata-sphinx-theme:

And from docusaurus: