Added netezza in external dialects in documentation

Added netezza in external dialects in documentation

### Description
IBM Netezza Performance Server (aka Netezza, NPS) now has fully function SQLAlchemy dialect. Need to add it in SQLAlchemy 1.3 documentation.
This PR fixes https://github.com/sqlalchemy/sqlalchemy/issues/6609

### 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: #6610
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/6610
Pull-request-sha: 2de51ff3887048e7df39e71b30b0e1824cfa0f56

Change-Id: I3a3f49981a4aaca0874bc2765cd782da61a258ef