Docs: suggestions for avoiding PII in docs

What does this MR do?

This MR proposes a new section in the GitLab style guide recommending against using personally identifying information (PII) in code examples.

• It recommends using email addresses ending in example.com
• It recommends using obviously-fake usernames like Example User, with a backup solution of using non-gendered names with common surnames, such as Sidney Jones or Dana Doe.

(I actually feel pretty strongly about the second item - in examples, names are often gendered so that traditionally male names are used in developer documentation, and traditionally female names are used in marketing- and UX-related examples.)

cc @gl-docsteam to get feedback from the rest of the team.

Related issues

• Closes #208372 (closed)
• Some initial cleanup was done in !26133 (merged)

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• If applicable, update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.