Move `CONTRIBUTING.md` to `docs/`, review and update text, and add pipline-specific conventions section

[jfy133]

This PR partly addresses as first step https://github.com/nf-core/website/issues/3653

1. Moves location of CONTRIBUTING.md to docs/ - to centralise all docs that should be displayed on the website for a pipeline
2. Adds website-compatible astro header for more interactive checklist compatibility
3. Reordering to better follow the order described in the different checklists
4. Generally reviewed text and formatting
5. Converted lists to checklists
6. Checked all links were up-to-date and updated where broken
7. Added a customisable section at the end for pipeline devs to describe specific contribution procedures

PR checklist

• [x] This comment contains a description of changes (with reason)
• [ ] CHANGELOG.md is updated
• [ ] If you've fixed a bug or added code that should be tested, add tests!
• [ ] Documentation in docs is updated

[jfy133]

@christopher-hakkaart if you have an early version of a style guide, maybe we can use this page as a guinea pig (which doesn't fall in the website so it sort of falls out of our primary remit)?

[christopher-hakkaart]

The text could be improved - mostly for consistency and removing unnecessary words to enhance clarity.

The early style guide is linked below, but is subject to change.

https://docs-v2--nf-core-docs.netlify.app/docs/contributors/documentation

[jfy133]

The text could be improved - mostly for consistency and removing unnecessary words to enhance clarity.

The early style guide is linked below, but is subject to change.

https://docs-v2--nf-core-docs.netlify.app/docs/contributors/documentation

(self note) style guide: https://docs-v2--nf-core-docs.netlify.app/docs/developers/documentation/style_guide

[christopher-hakkaart]

(self note) style guide: https://docs-v2--nf-core-docs.netlify.app/docs/developers/documentation/style_guide

Argh - yeah that was my bad sorry, added the link and moved the page 20 min later 😅