Lint documentation with Vale

What does this merge request do and why?

This MR adds documentation linting with Vale. This is to keep the documentation consistent across GitLab projects as per the guidelines and improve doc review efficiency.

• GitLab styles are lifted from GitLab project.
• markdown-lint takes care of other rules and will be added in a separate MR to reduce the MR size.

The documentation lint can be run via:

• Manually run via make lint-doc.
• Automatically via lefhook pre-commit and pre-push hook.
• Checked via CI in lint:doc job.

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

• I've ran the affected pipeline(s) to validate that nothing is broken.
• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.