feat(self-hosted): Add docs around mail setup (getsentry#129)

See https://app.asana.com/0/1169344595888357/1190341230301191/f.

Closes getsentry/self-hosted#565

src/components/sidebar.js

@@ -89,6 +89,7 @@ const Sidebar = () => (
       <ul className="list-unstyled" data-sidebar-tree>
         <NavLink to="/self-hosted/">Overview</NavLink>
         <NavLink to="/self-hosted/releases/">Versioning & Releases</NavLink>
+        <NavLink to="/self-hosted/mail/">Mail</NavLink>
         <NavLink to="/self-hosted/support/">Support</NavLink>
       </ul>
     </li>

src/docs/self-hosted/mail.mdx

@@ -0,0 +1,17 @@
+---
+title: 'Self-Hosted Mail'
+---
+
+<Alert title="Note" level="info">
+    Remember that you'd need to restart all Sentry services once you change your settings. See the <Link to="/self-hosted/#configuration">configuration section</Link> for more information.
+</Alert>
+
+## Outbound Mail
+
+Self-hosted Sentry ships with a built-in outgoing SMTP server powered by [exim4](https://hub.docker.com/r/tianon/exim4). The default configuration is set to use this server. All you need to do is to set a valid address for `mail.from` setting in `config.yml` . Keep in mind that if you start sending too many emails to public addresses, your new server may get marked as a spammer and banned.
+
+If you want to use an external SMTP server you can set the relevant `mail.*` settings in `config.yml` file and ignore the built-in SMTP server. Refer to our [email service documentation](/services/email/) for all the details on what each setting means and does.
+
+## Inbound Mail
+
+Sentry has a very limited inboud mail support through [Mailgun](https://documentation.mailgun.com/en/latest/quickstart-receiving.html). You can find all the information regarding how to set this up over at our [inbound email service documentation](/services/email/#inbound-email).