Document how to create a plugin for other database dialects

[papb]

Issue Description

What was unclear/insufficient/not covered in the documentation

If a developer is willing to develop support for another dialect for Sequelize, what are the steps to do it?

Additional context

PR sequelize/sequelize#11073 is an example of attempt to add support for a new dialect to Sequelize. However we should have a guide on how to do it, and ideally should offer a way for users to write separate plugins to do this, so that they don't have to go over the work of a big PR.

Issue Template Checklist

Is this issue dialect-specific?

• [X] No. This issue is relevant to Sequelize as a whole.
• [ ] Yes. This issue only applies to the following dialect(s): XXX, YYY, ZZZ
• [ ] I don't know.

Would you be willing to resolve this issue by submitting a Pull Request?

• [ ] Yes, I have the time and I know how to start.
• [ ] Yes, I have the time but I don't know how to start, I would need guidance.
• [X] No, I don't have the time, although I believe I could do it if I had the time...
• [ ] No, I don't have the time and I wouldn't even know how to start.

[github-actions[bot]]

This issue has been automatically marked as stale because it has been open for 7 days without activity. It will be closed if no further activity occurs. If this is still an issue, just leave a comment or remove the "stale" label. 🙂