[ci,nb] Refactored notebook testrunner.

Another notebook testrunner refactoring...

• Simplified logic.
• All notebook formats now use regular TOML-frontmatter (with +++-delimiter), no more <!--eofm--> hacking.
• Removed nb2hugo dependency.
• Adapted notebooks to the new frontmatter format.
• When using the ci::web only-label regular benchmarks are skipped which improves CI runtime quite a bit.

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behavior change was documented?