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