Add tasks docs_info

[jluethi]

Draft of polished tasks_info for each of the core tasks.

@tcompa This is to address #874 (comment)

While setting it up, I started liking the pattern of mostly defining Purpose & Limitations, but some tasks get extra info. This shows for me that it's indeed valuable to use markdown files, not just strings (as you suggested in the issue).

Can we adapt the manifest building to load the file content if a task contains something like docs_info="file:task_info/convert_cellvoyager_to_ome_zarr.md"?

Checklist before merging

• I added an appropriate entry to CHANGELOG.md

[github-actions]

Coverage report

This PR does not seem to contain any modification to coverable code.

[tcompa]

This is ready, from my side.

A couple of minor remarks:

1. I had to open&fix Deprecate check_manifest.py (in favor of recreate+diff) #877
2. I introduced support for docs_info being set to file:XYZ, where XYZ is a relative path. The absolute path of the docs file is then built by combining the base folder where task_list.py lives with this XYZ relative path.

[jluethi]

@tcompa That sounds good to me :)
I added a little bit of extra formatting (making some text elements bold). With that, I'd say we can merge & release that. I'll then add that to the template later & start making another round of PRs to repos after that.