Add self-service player deletion link tip

[tudddorrr]

No description provided.

[claude]

This PR adds a useful tip callout in both the Godot and Unity player authentication docs, surfacing the self-service deletion link feature. Small, well-placed, and a genuine improvement to developer experience. One minor note below.

Minor: Clarity

"You can provide your players with a self-service deletion link" is slightly ambiguous about what form this link takes. A developer reading this might wonder whether it is a URL to display in-game, share via email, or something else entirely.

Suggested rewrite: "You can provide your players with a self-service account deletion link — a URL they can visit to delete their own account. Find the link on your game's settings page."

This applies identically to both the Godot and Unity versions of the tip.

Everything else looks good

• Tip placement (after the deletion code block, before the Deleted aliases section) is logical
• External dashboard URL is appropriate here
• The tip callout type is correct — this is optional info, not a warning
• Changes are consistent across both Godot and Unity docs