Add & update security considerations for PATs

What does this MR do?

Access tokens must be treated carefully. We have provided guidance on this, but it is not easily discoverable from the page where users are shown how to create Project access tokens.

This MR introduces a warning box which links to security considerations, and briefly summarises them, to encourage our users to follow best practice.

Related issues

• One of the Import group's security threats is long-lived tokens. This discussion here kicked off this MR, about encouraging users to consider short-lived ones. https://gitlab.com/gitlab-org/manage/import/backend/discussion/-/issues/9#note_719987300

Author's checklist

• Consider taking the GitLab Technical Writing Fundamentals course
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• Ensure that the product tier badge is added to topic's h1.
• Request a review based on:
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  • The associated Technical Writer.

Review checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
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    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
• Ensure a release milestone is set.