Breadcrumbs use absolute links

For example, visit https://grpc.io/docs/languages/, and inspect the breadcrumb for **Documentation**. This is the HTML associated with the breadcrumbs:

```html

1. Documentation


2. Languages

```

This isn't an issue pre se, just a bit surprising: and it burdens the link checker when checking external links. Since we're already overriding the layout, we could switch to using relative links.

Resources:
- https://github.com/google/docsy/blob/master/layouts/partials/breadcrumb.html, which uses `.Permalink`
- https://gohugo.io/content-management/page-resources/#properties
- https://www.w3.org/wiki/WebSchemas/Breadcrumbs **seems to imply that relative links are ok**
- https://schema.org/BreadcrumbList
- You can test your pages here: https://developers.google.com/search/docs/advanced/structured-data
The Flutter site uses relative URLs and passes Google Rich Results Test and the Schema Markup Validator.