[FIX] mail: modify url redirection to doc (mail servers)

Documentation structure were modified to point out to a new subtree:
    content/applications/general/email_communication/email_server

Related task:2619564
Documentation PR:1092

closes odoo/odoo#84548

X-original-commit: 8e57b42f
Signed-off-by: Vergote Baptiste (bve) <bve@odoo.com>