Add new BackendTLSPolicy configuration options to documentation

[08volt]

/kind documentation

What this PR does / why we need it: Updated documentation page regarding BackendTLSPolicy with the following fields:

• Gateway backendTLS field
• subjectAltNames field
• options field

The documentation includes descriptions of each new field along with their purpose, usage constraints and reference links.

Fixes # Does this PR introduce a user-facing change?:

NONE