chore: lint documentation with vale

What does this merge request do and why?

This MR adds documentation linting with Vale.

• GitLab styles are lifted from GitLab CLI project.
• Spelling exceptions is excluded since I think it is a bit excessive as none applied to this repo and can be added on a case-by-case basis.
• markdown-lint takes care of other rules and will be added in a separate MR to reduce the MR size.

How to set up and validate locally

1. Check out to this merge request's branch.
2. Run the command to lint docs.

   make lint-doc

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.