Omit Operator design records

What does this MR do and why?

Removes pages under /operator/adr from the Docs build. These are architectural design records and not needed with general documentation.

Closes #1275 (closed)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. Compile the site and verify ADR pages like this are not accessible:

• http://sarahg-1275-exclude-operator-adr.35.193.151.162.nip.io/operator/adr/0010-ci-environments.html
• http://sarahg-1275-exclude-operator-adr.35.193.151.162.nip.io/operator/adr/0015-ci-pipelines-in-dev.html

Note: in the review app, 404s redirect to the homepage. Locally, you'll see an unstyled error page. In production, we should see the styled 404 page. Always good to keep things interesting.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.