Documentation - updated email setup (#10695)

* Updated email setup documentation to include sendmail option

* Update docs/content/doc/usage/email-setup.en-us.md

full path to sendmail

Co-Authored-By: mrsdizzie <info@mrsdizzie.com>

* Update docs/content/doc/usage/email-setup.en-us.md

docker image does not have sendmail

Co-Authored-By: Antoine GIRARD <sapk@users.noreply.github.com>

* Update docs/content/doc/usage/email-setup.en-us.md

Co-Authored-By: Antoine GIRARD <sapk@users.noreply.github.com>

* Update docs/content/doc/usage/email-setup.en-us.md

Co-Authored-By: James Lakin <jamesorlakin@users.noreply.github.com>

* Update docs/content/doc/usage/email-setup.en-us.md

Co-Authored-By: James Lakin <jamesorlakin@users.noreply.github.com>

Co-authored-by: 8ctopus <hello@octopuslabs.io>
Co-authored-by: mrsdizzie <info@mrsdizzie.com>
Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Antoine GIRARD <sapk@users.noreply.github.com>
Co-authored-by: James Lakin <jamesorlakin@users.noreply.github.com>

docs/content/doc/usage/email-setup.en-us.md

@@ -15,19 +15,33 @@ menu:
 
 # Email setup
 
-- To use Gitea's built-in Email support, update the `app.ini` config file [mailer] section:
+To use Gitea's built-in Email support, update the `app.ini` config file [mailer] section:
 
+## Sendmail version 
+Use the operating system’s sendmail command instead of SMTP. This is common on Linux servers.  
+Note: For use in the official Gitea Docker image, please configure with the SMTP version.
 ```ini
 [mailer]
-ENABLED = true
-HOST    = mail.mydomain.com:587
-FROM    = gitea@mydomain.com
-USER    = gitea@mydomain.com
-PASSWD  = `password`
+ENABLED       = true
+FROM          = gitea@mydomain.com
+MAILER_TYPE   = sendmail
+SENDMAIL_PATH = /usr/sbin/sendmail
+```
+
+## SMTP version
+```ini
+[mailer]
+ENABLED        = true
+FROM           = gitea@mydomain.com
+MAILER_TYPE    = smtp
+HOST           = mail.mydomain.com:587
+IS_TLS_ENABLED = true
+USER           = gitea@mydomain.com
+PASSWD         = `password`
 ```
 
 - Restart Gitea for the configuration changes to take effect.
 
 - To send a test email to validate the settings, go to Gitea > Site Administration > Configuration > SMTP Mailer Configuration.
 
-For the full list of options check the [Config Cheat Sheet]({{< relref "doc/advanced/config-cheat-sheet.en-us.md" >}})
+For the full list of options check the [Config Cheat Sheet]({{< relref "doc/advanced/config-cheat-sheet.en-us.md" >}})