feat(usermanual): One-click unsubscribe for Mail

[ChristophWurst]

☑️ Resolves

• Fixes One Click Suggested Unsubscribe mail#5387
• Fixes Document one-click suggested unsubscribe for Mail #10015

🖼️ Screenshots

[ChristophWurst]

/backport to stable26

[backportbot-nextcloud]

The backport to stable26 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable26
git pull origin/stable26

# Create the new backport branch
git checkout -b fix/foo-stable26

# 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-stable26

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

[ChristophWurst]

#9925 was missing for stable26

[ChristophWurst]

/backport to stable26