docs: rewrite email setup (#16404)
docs: rewrite email setup (#16404)

* Add intro for both the docs page and mailer methods
  * Fix numbering level in SMTP section
  * Recommends implicit TLS

Signed-off-by: Bagas Sanjaya <818e5151593b2d2687964f0b8c2b102b8b80928c@gmail.com>
go-gitea/gitea,go-gitea/gitea,go-gitea/gitea