doc change - Add new external dialect for openGauss

doc change - Add new external dialect for openGauss

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### Description
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Added new external dialect for [openGauss](https://www.opengauss.org/en/).

### Checklist
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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
(cherry picked from commit db6145287244ca9801f7b7e1acc90cd25a572cde)