update output docs with info about custom multiqc sections

[pontushojer]

Relates to: #196

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
• If necessary, also make a PR on the nf-core/seqinspector branch on the nf-core/test-datasets repository.
• 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).

[Copilot]

Pull request overview

This PR updates the pipeline output documentation to better describe custom sections that nf-core/seqinspector adds to MultiQC reports, addressing gaps noted in issue #196.

Changes:

• Added documentation for the MultiQC subsampling notice section.
• Added documentation (including a screenshot) for the in-report MultiQC reports index.
• Added a corresponding entry to CHANGELOG.md.

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 2 comments.

File	Description
docs/output.md	Adds new documentation for custom MultiQC sections (subsampling notice and report index).
CHANGELOG.md	Records the documentation update under the dev “Added” section.

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