feat(documentation): explain custom/new postgres data types

[javiertury]

Pull Request check-list

• Does npm run test or npm run test-DIALECT pass with this change (including linting)?
• Does the description below contain a link to an existing issue (Closes #[issue]) or a description of the issue you are solving?
• Have you added new tests to prevent regressions?
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Description of change

Issue #10114. I propose to document how to add custom data types. In this pull request I illustrate only how to add postgres data types.

[codecov]

Codecov Report

Merging #10170 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #10170   +/-   ##
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  Coverage   96.34%   96.34%           
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  Files          63       63           
  Lines        9420     9420           
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  Hits         9076     9076           
  Misses        344      344

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[sushantdhiman]

I suggest create a new page called Datatypes, move stuff from https://github.com/sequelize/sequelize/blob/master/docs/models-definition.md#data-types and then add a section about customization

[javiertury]

That part is done. I did not find tests for documentation, if such a thing exists. I have just used grep to find a repair broken links.

[sushantdhiman]

Begin with # Datatypes, so nav item is also named as Datatypes

[javiertury]

Done.

[sushantdhiman]

Thanks @javiertury

[janmeier]

👍 I've been meaning to write something like this for ages! :)