Adds docs for the breadcrumbs block

[thijsoo]

Fixes #293

Summary

Relevant technical choices

Technical choices that affect more than this issue:

Test instructions

This PR can be acceptance tested by following these steps:

• Go to /features/blocks/breadcrumbs/
• Make sure the docs make sense :)

Quality assurance

• [X] Security - I have thought about any security implications this code might add.
• [ ] Performance - I have checked that this code doesn't impact performance (greatly).
• [ ] Caching - I have analyzed the caching methods that this code touches and have added instructions to deal with those.
• [ ] Tested - I have tested this code to the best of my abilities.
• [ ] Automated tests - I have added unit tests to verify the code works as intended.
• [ ] Testability - I have added unique ids to elements, so they can be located in automated testing.
• [ ] I have altered a filename.
  • [ ] I have adjusted the ID property accordingly and updated all internal links.
  • [ ] I have added the redirect to the _redirects file in the root of the project.

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