Add support for displaying `CONTRIBUTING.md` on the nf-core website

[jfy133]

Is your feature request related to a problem? Please describe.

The CONTRIBUTING.md file is currently under utilised in nf-core pipelines.
It could be very useful for larger (VIP?) pipelines to document pipeline-specific development protcools and procedures, particularly if it includes interactive checklists etc. (e.g. https://nf-co.re/createtaxdb/dev/docs/usage/dev), however it's currently hidden away and not very visible

Describe the solution you'd like
The contributing.md page to be displayed on the website on pipeline pages, to ensure we retain a 'healthy' GitHub repository status, but can promote greater usage of the file.

Describe alternatives you've considered
Just assume developers will be aware of the file on the github repository.

Additional context
Add any other context or screenshots about the feature request here.

[jfy133]

I just noticed however the webstie does not appear to detect GitHub-flavoured markdown checklists as proper checklists 🤔, so would need some advice there

[ewels]

Checklists: https://nf-co.re/docs/contributing/website/markdown#checklist (this is a good page for you to have bookmarked now that you're on the @nf-core/documentation team! 😉 )

[jfy133]

Checklists: https://nf-co.re/docs/contributing/website/markdown#checklist (this is a good page for you to have bookmarked now that you're on the @nf-core/documentation team! 😉 )

I meant more whether the GitHub version would work with the astro (i presume?) header, but noted ;)