Port agent documentation help link from Kea (main) to Swift

[florian-muller]

The agent documentation help link feature (see #1909) landed on Kea (main) via #1910. It needs to be ported to the swift branch (cherry-pick if appropriate).

This issue only tracks the port — no implementation details needed here.

• Source: Add a documentation help link on the agents page #1909 / feat(kf): add optional agent documentation help link #1910 (Kea / main)
• Target: swift branch