Add redirect and renaming note for exporter

What does this MR do?

When GitLab Monitor Exporter was renamed in !16511 (merged), the old file was deleted instead of adding a redirect, which broke links to it from outside projects (Omnibus docs, for example). Found while working on: gitlab-docs#436 (closed)

I added the redirect per https://docs.gitlab.com/ee/development/documentation/#changing-document-location. I also expanded the note to explain the renaming, so that it is still findable when searching for the old name (I hope)

Related issues

Closes #32312 (closed)

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 and that you merge the equivalent EE MR before the CE MR if both exist.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.