Sendmail should create a process on the gitea system and have a default timeout (#11256)

* Make sure that sendmail processes register with the process manager
* Provide a timeout for these (initially of 5 minutes)
* Add configurable value and tie in to documentation
* Tie in to the admin config page.

Signed-off-by: Andrew Thornton <art27@cantab.net>

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -410,6 +410,7 @@ set name for unique queues. Individual queues will default to
    - Enabling dummy will ignore all settings except `ENABLED`, `SUBJECT_PREFIX` and `FROM`.
 - `SENDMAIL_PATH`: **sendmail**: The location of sendmail on the operating system (can be
    command or full path).
+- `SENDMAIL_TIMEOUT`: **5m**: default timeout for sending email through sendmail
 - ``IS_TLS_ENABLED`` :  **false** : Decide if SMTP connections should use TLS.
 
 ## Cache (`cache`)