docs: use folder terminology in mail manual

[MukundaKatta]

Summary

• update the Mail user manual to use folder terminology instead of mailbox terminology
• keep the existing shared mailbox icon asset reference intact while updating visible text

Fixes #13898

Testing

• git diff --check
• verified referenced shared mailbox icon file exists

Note: full Sphinx build was not run locally because Sphinx is not installed in this environment.

[MukundaKatta]

Pushed 310676b to fix the DCO failure.

Root cause: the single PR commit was missing the required Signed-off-by trailer. I amended the commit with signoff and force-pushed with lease. The documentation diff itself is unchanged.

Validation run locally:

• git diff --check
• verified the amended commit contains Signed-off-by

[ChristophWurst]

The text and the subline of a heading has to match in length

Please build the documentation locally and check any warnings and errors

Also post a screenshot of the HTML manual with your applied changes

Thanks for the contribution

[welcome]

Thanks for your first pull request and welcome to the community! Feel free to keep them coming! If you are looking for issues to tackle then have a look at this selection: https://github.com/nextcloud/documentation/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22