BS3: Add third party support docs

Fixes/addresses #8945 and I imagine a few other issues.

I'd like to address third-party (in)compatibility issues more directly in the v3 docs. While we can't obviously support All the Things™, we *can* document some known issues and help guide folks to the right solutions. The first one up is Google Maps. Once this is merged, we can consider other additions.

For the record, the `.img-responsive` in v3 fixes nearly all other map-product issues, so that's boss.

Future additions should not be extensive guides, but rather a quick explanation of the problem and how to solve it, perhaps with a code snippet, and most definitely with a link for more information.

/cc @cvrebert