Documentation: Write TLS Passthrough how-to

[mpstefan]

As a user of NGF
I want a guide that shows me how to enable TLS Passthrough for my route
So that I can ensure my incoming traffic is only decrypted by my application
And so that I do not need to lookup Gateway API reference documentation
And so I know that TLS Passthrough is a feature NGF has.

Acceptance

• A guide is written on how to configure TLS passthrough for an application accessible by NGF.
• Update compatibility doc