Add link to editor a11y guide

What does this MR do and why?

The pipeline editor uses Monaco text editor. Monaco has some great a11y features built in, including a mechanism for escaping keyboard traps.

The user is not made aware that it uses the Monaco text editor, so is not aware of the a11y features. This can lead to a user becoming keyboard trapped using the tab key.

This MR links to the a11y guide for the editor

References

• https://gitlab.com/gitlab-org/gitlab/-/issues/511809+

Screenshots or screen recordings

Before	After

How to set up and validate locally

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