docs: Update certificate loading documentation

[goatgoose]

Description of changes:

This PR updates the following documentation related to loading certificates:

• Adds an SNI section to the usage docs, discussing what the SNI extension is and how s2n-tls selects certificates according to the domains that certificates are issued for.
• Updates the API documentation for the trust store loading APIs to provide information on the PEM parsing implementation used in each API, and provides guidance on which should be used.

Call-outs:

Let me know if anything is confusing!

Testing:

Documentation change.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.