Document options for breadcrumbs

[chalin]

Related:

• https://github.com/search?q=repo%3Agoogle%2Fdocsy+breadcrumbs+is%3Aopen
• Support hiding single/root breadcrumbs #786
• Breadcrumb fix for sites with docs at the root #807
• Show breadcrumbs on mobile too #784: document how to hide on mobile for backwards compatibility? (Not sure it's a good idea to do so, but I still wanted to make note of it.)