MINOR: Add Exactly-once support for connectors to the docs table of contents

[yashmayya]

• Documentation for exactly-once support for connectors was recently added in KAFKA-13709: Add docs for exactly-once support in Connect #12941
• Although it's a top level header under KAFKA CONNECT -> User Guide, adding an entry to the docs table of contents was missed.

Before:

After:

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[yashmayya]

cc @C0urante

[C0urante]

Thanks Yash!