Update docs

[maxulysse]

Many thanks to contributing to nf-core/sarek!

• Add location for abstracts
• Remove reference to old build.nf script which is now part of main.nf

Please fill in the appropriate checklist below (delete whatever is not relevant).
These are the most common things requested on pull requests (PRs).

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 necessary, also make a PR on the nf-core/sarek branch on the nf-core/test-datasets repo
• Ensure the test suite passes (nextflow run . -profile test,docker).
• Make sure your code lints (nf-core lint .).
• Documentation in docs is updated
• CHANGELOG.md is updated
• README.md is updated

Learn more about contributing: guidelines