Vale for prose checking

[regularfry]

Description

Vale is a tool for performing a certain set of style checks, which this PR introduces for markdown files only. Vale can scan code comments, but as an initial validation we should limit the scope.

WIP for the moment while I sort out github action integration.

Context

We want to be sure that our documentation is consistent and readable.

Type of changes

• [ ] Refactoring (non-breaking change)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would change existing functionality)
• [ ] Bug fix (non-breaking change which fixes an issue)

Checklist

• [x] I am familiar with the contributing guidelines
• [x] I have followed the code style of the project
• [ ] I have added tests to cover my changes
• [x] I have updated the documentation accordingly
• [ ] This PR is a result of pair or mob programming

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Sensitive Information Declaration

To ensure the utmost confidentiality and protect your and others privacy, we kindly ask you to NOT including PII (Personal Identifiable Information) / PID (Personal Identifiable Data) or any other sensitive data in this PR (Pull Request) and the codebase changes. We will remove any PR that do contain any sensitive information. We really appreciate your cooperation in this matter.

• [x] I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.

[regularfry]

I've double-checked that we pick up vale failures with a test PR here. That was successful, so I think we can say this is no longer WIP.