docs: fix syntax highlighting

[ShortDevelopment]

We're using openqasm-pygments for syntax highlighting openqasm3 in our docs. As this package contains a bug in lexing indented annotations, we're now using a custom version until the upstream PR is merged.

Definition of Done

• Code Completion: All necessary code for the feature or bug fix has been written.
• Testing: Relevant tests (unit, integration, user acceptance) have passed.
• Peer Review: The code has undergone peer review by another team member.
• Documentation: Required documentation, such as comments and API documentation, is complete.
• Deployment Readiness: The work is ready for deployment to production if needed.

[arne-gbr]

Nice!