doc change - Add new external dialect for openGauss

doc change - Add new external dialect for openGauss

<!-- Provide a general summary of your proposed changes in the Title field above -->

### Description
<!-- Describe your changes in detail -->
Added new external dialect for [openGauss](https://www.opengauss.org/en/).

### Checklist
<!-- go over following points. check them with an `x` if they do apply, (they turn into clickable checkboxes once the PR is submitted, so no need to do everything at once)

-->

This pull request is:

- [x] A documentation / typographical error fix
	- Good to go, no issue or tests are needed
- [ ] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [ ] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

**Have a nice day!**

Closes: #8803
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/8803
Pull-request-sha: ca5a7f1310b2ce93ce33618e0609abe23b41a3da

Change-Id: Ia8ca38f98f346fa3cc910ceb7af47f8c903eb587