Add Vale linting to docs pipeline

What does this Merge Request do?

Follows on from: !1141 (merged) that enabled markdownlint.

This MR would raise the level of docs linting to the same as our published product documentation.

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.