ENH: Move freesurfer/fastsurfer output directory alongside output dir for easier reusability

[gagnonanthony]

Freesurfer outputs are not BIDS-compliant, which breaks the current output file structure. Similar to other pipelines, the output directory for freesurfer/fastsurfer is moved to its dedicated output folder (alongside the nf-pediatric output folder), enhancing reusability in other contexts.

PR checklist

• This comment contains a description of changes (with reason).
• If you've fixed a bug or added code that should be tested, add tests!
• If you've added a new tool - have you followed the pipeline conventions in the contribution docs
• Make sure your code lints (nf-core pipelines lint).
• Ensure the test suite passes (nextflow run . -profile test,docker --outdir <OUTDIR>).
• Check for unexpected warnings in debug mode (nextflow run . -profile debug,test,docker --outdir <OUTDIR>).
• Usage Documentation in docs/usage.md is updated.
• Output Documentation in docs/output.md is updated.
• CHANGELOG.md is updated.
• README.md is updated (including new tool citations and authors/contributors).