Extend YAML validation for use on Walkthroughs

What does this MR do and why?

• Refactors the YAML schema validation script we use for the global nav (navigation.yaml) so that it can be used for any set of YAML files + schema files
• Adds a schema definition for walkthrough YAML
• Adds a CI job for YAML validation (and adjusts the existing navigation.yaml schema test to run here)

Relates to #298

Screenshots, screen recordings, or links to review app

Successful job on CI: https://gitlab.com/gitlab-org/technical-writing/docs-gitlab-com/-/jobs/10202911290

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment.
2. Check out this branch and run make setup to update dependencies.
3. Run make validate-yaml. Tests should pass.
4. Edit data/en-us/navigation.yaml and add a new property with an invalid name, like:

  url: 'user/get_started/'
+ hotdogs: 'yes'

2. Run make validate-yaml. Tests should fail.
3. Repeat with a file in data/en-us/walkthroughs. Try removing a required property, like:

- entry_point: "first_step_id"

4. Run make validate-yaml. Tests should fail.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.