It would be great to show "how to contribute to documentation" in the "Non Code Related" section of the "Contribute" page.

The related information is already in the "Code documentation" subsection in the "Code Related" section. Surely, code documentation needs code experience, but the documentation on this site doesn't necessarily need code experience. So non code related section is a good place to put this kind of information in, I think. It would be easier for new contributors to find.

The most of Redmine documentation is written on the "Wiki" of redmine.org. So, that is better to have a link to "Submitting a Wiki improvement" subsection in the "Submissions" page in this section.

For example,

How to contribute to documentation on this site

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You can contribute to the documentation on this site with directly editing the Wiki. Firstly you need to register your account on this site and you can edit the Wiki. Some Wiki pages doesn't allow you to edit and in the case you need to file an issue on the issue queue with the category "documentation".