Documentation: Add eturnal as a second TURN server example

[sando38]

☑️ Resolves

• Follow up Documentation: Provide a second TURN server example (eturnal) #9816

🖼️ Screenshots

🏚️ Before	🏡 After
One document doc/TURN.md	Three documents: doc/TURN.md as overview, doc/coturn.md & doc/eturnal.md

🚧 Tasks

• Review the changes
• Check if links still work
• Check if doc/index.md is correct

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not required
• 📘 API documentation in docs/ has been updated or is not required
• 📗 User documentation in https://github.com/nextcloud/documentation/tree/master/user_manual/talk has been updated or is not required
• 🔖 Capability is added or not needed

Message

Please have a look into the changes of the documentation page.
Please also see, if mentioning the prometheus exporter is something to be added into the documentation:
processone/eturnal#41

[nickvergessen]

Otherwise fine and we are ready to go

[nickvergessen]

/backport to stable27

[backportbot-nextcloud]

The backport to stable27 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable27
git pull origin stable27

# Create the new backport branch
git checkout -b fix/foo-stable27

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts. Resolve them.
git cherry-pick abc123

# Push the cherry pick commit to the remote repository and open a pull request
git push origin fix/foo-stable27

More info at https://docs.nextcloud.com/server/latest/developer_manual/getting_started/development_process.html#manual-backport

[nickvergessen]

Wrong tab