docs: document integrations fork mirroring setup

What does this MR do?

When I initially create the GitLab UI Integrations fork, I configured it to pull-mirror the GitLab repo with my own user account. This resulted in my contributions graph getting polluted with all those automated events:

Hey, I was OOO all of last week!

As much as I like seeing those shiny white squares, I don't want these mirroring events to count towards my contributions. It also simply feels like bad practice for a team member's account to be responsible for this sort of tasks. What happens if I quit (god forbid)?

Anyways, I re-configured the mirroring using a Project Access Token so that the process is completely separated from my account. This will require that we rotate the token every year, so I'm documenting the process here.

Screenshots or screen recordings

Integration merge requests

• GitLab: mr_url
• CustomersDot: mr_url
• Status Page: mr_url

Does this MR meet the acceptance criteria?

This checklist encourages the authors, reviewers, and maintainers of merge requests (MRs) to confirm changes were analyzed for conformity with the project's guidelines, security and accessibility.

Toggle the acceptance checklist

Conformity

• Code review guidelines.
• GitLab UI's contributing guidelines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened (see integration merge requests above).
• Added the ~"component:*" label(s) if applicable.

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
• Security reports checked/validated by a reviewer from the AppSec team

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.