Initial a11y scanning CI template

What does this MR do?

This MR adds a CI template in the Verify section with a basic implementation of Accessibility scanning using Pa11y.

• Installs pa11y@5.3.0 and pa11y-reporter-html@1.0.0
• The template and configuration is modeled after Browser Performance Testing
• The job outputs an artifact called accessiblity.html that is browsable from the job artifacts page.
• The currently static configuration includes warnings and notices.
• Refactor the way we keep the Verify/Browser-Performance template out of FOSS to allow for Verify/Accessibility to be made available.

Related to: #25566 (closed)

What does this MR not do?

• Create a Verify/Accessibility Docker image with all the required software pre-installed. Setting this up as a separate project will cut down on the CI minutes required to produce a report.
• Specify package.json for the installation, that will go in the Verify/Accessibility Docker image.
• Output a JSON-formatted job artifact for API/machine consumption.
• Set up a11y to automatically scan a review app
• Kubernetes?

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers](https://docs.gitlab.com/ee/install/requirements.html#supported-web-browsers)
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• ~~Label as security and @ mention @gitlab-com/gl-security/appsec~~
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team