Update Universal Link documentation

[pyby]

Description

#439 switched connection of non-production builds to Play web staging domain.
The documentation was not updated throw.

This PR proposes to update and improve the documentation.

Changes Made

• Update CUSTOM_URLS_AND_UNIVERSAL_LINKS.md.
  • Mention of Play web stating instead of Play MMF for test purposes.
  • Improve the how to test steps.

Checklist

• I have followed the project's style guidelines.
• I have performed a self-review of my own changes.
• I have made corresponding changes to the documentation.
• My changes do not generate new warnings.
• I have tested my changes and I am confident that it works as expected and doesn't introduce any known regressions.
• I have reviewed the contribution guidelines.