feat(docs): Add vale style checker

[merkata]

Applicable spec:

Overview

Adds the vale action with the Canonical vale styles.

The Canonical styles are hosted at praecepta and we're making use of the recommended workflow that is supplied in the README.md. In addition to our operator-workflows, data-platform-doc-tools is using the same workflow.

The vale action being used is the official one.

Rationale

Vale is gaining adoption as a doc style checker, hence including it in our workflow to test things out (currently it will not fail the action).

Workflow Changes

The test.yaml contains a style checker job after woke.

Checklist

• [x ] The contributing guide was applied
• [x ] The PR is tagged with appropriate label (urgent, trivial, complex)

[merkata]

@arturo-seijas @erinecon @gregory-schiano this is mergeable now, note that for github runners I have to explicitly install tree and ruby-rubygems for now. Using this for a while should give us insights into what to improve in the docs/ folders of our charms and also contribute back to https://github.com/canonical/praecepta that hosts the Canonical styles.
Idea is to test this early on and see how that works, no roadmap item or anything, pure dev fun atm.