Add admin > mailer page

[j0k3r]

Also lock node version to allow gitbook to work

You can see the new page here: https://deploy-preview-178--wallabag-doc.netlify.app/en/admin/mailer

See:

• https://github.com/wallabag/wallabag/issues/6898
• https://github.com/wallabag/wallabag/pull/6899

[nicosomb]

👏