Clarify module variable cleanup

[El-Limon]

Summary

This updates the module docs to make variable cleanup behavior explicit. Uninstalling a module removes the game server installation and config, but it does not delete variables created by that module.

The change also points users to the Advanced Variables deletion pattern and adds an example for deleting module-scoped variables by ID.

What Changed

• Module install docs: Clarified that uninstalling removes the installation/configuration only, and that module variables remain until deleted separately.
• Module debugging docs: Added a pointer to delete relevant variables when saved module data needs cleanup.
• Advanced Variables docs: Added a module cleanup example that searches by gameServerId and moduleId, then deletes each variable by ID.

Reviewer Guide

• Start here: docs/modules/install-configure.md, especially the "Remove a module" section.
• Pay attention to: The wording around uninstall behavior. It should match the service behavior where uninstall deletes the installation row, not variables.

Testing Plan

Happy Path

• Open the Modules install/configure page and verify the uninstall section clearly says variables are not deleted.
• Open Advanced > Variables and verify the deletion example explains search-then-delete by variable ID.

Regression Checks

• Confirm links to /advanced/variables#deleting-a-variable resolve.
• Confirm the docs build passes.

Verified locally with npm run build.