Add section on 'truststore' feature to User Guide

[sethmlarson]

As promised in #11082, here is the user guide section for the truststore feature flag. Let me know what additional information I should add here.

cc @davisagli

[pradyunsg]

Some part of me wants there to be a topic page for certificate handling in pip -- similar to the authentication page we have today. This would involve moving all this content into a separate document (and anything else that's related), which can be done in a follow up.

[sethmlarson]

Thanks for the review and suggestion @pradyunsg! If desirable I can take a try at creating a separate "configuring certificates" page in another PR.

[sethmlarson]

Gentle ping on this one, let me know if there's more to do here. Would like to link to the docs when they're available! :)