Tutorials page alternative

[lordy5]

In this PR I created an alternative option for the tutorials index page. I think it is better than the previous option and has the following differences:

1. Broke up longer tutorial groups, like scRNA-seq into smaller subgroups, each with its own dropdown.
2. Used MyST cards instead of a list-table for each tutorial link. I think this looks less cluttered.
3. Used markdown headers so that users can easily navigate to the model type they want using the right sidebar.
4. Reordered tutorials and subgroups in a more logical way so that the page reads more like a tutorial.
5. Added descriptions for each subgroup so users have a better idea of whether those tutorials are relevant to them.

I think something like this layout is about as simplified as I can get with built in MyST while still allowing the user to skip to particular sections using the right sidebar. From the different options I've already tried, I think if there is an additional layer of subgroupings/labels/dropdowns then the page becomes too cluttered and is less helpful.

Still open for suggested changes and improvements of course.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.69%. Comparing base (8e95508) to head (0828988).

❗ There is a different number of reports uploaded between BASE (8e95508) and HEAD (0828988). Click for more details.

HEAD has 8 uploads less than BASE

Flag	BASE (8e95508)	HEAD (0828988)
	11	3

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3230      +/-   ##
==========================================
- Coverage   86.85%   80.69%   -6.16%     
==========================================
  Files         193      193              
  Lines       17080    17080              
==========================================
- Hits        14835    13783    -1052     
- Misses       2245     3297    +1052     

see 27 files with indirect coverage changes

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