Incorporate use case tutorials

[Lilly-May]

PR Checklist

• Referenced issue is linked (Feature pertpy use-cases in our tutorial #642)
• Documentation in docs is updated

Description of changes

• Added use case tutorials to the tutorial gallery
• Fixed a bug in the DE base class: it was using adata.layer (doesn't exist) instead of adata.layers, causing the code to fail whenever the layer parameter was specified

The Dialogue part is still missing from the Zhang tutorial, but I'll create a separate PR for that, as I'll have to run that part on the cluster.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 65.55%. Comparing base (6e3ca9c) to head (eff67ef).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...ertpy/tools/_differential_gene_expression/_base.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #665   +/-   ##
=======================================
  Coverage   65.55%   65.55%           
=======================================
  Files          47       47           
  Lines        6105     6105           
=======================================
  Hits         4002     4002           
  Misses       2103     2103           

Files with missing lines	Coverage Δ
pertpy/metadata/_cell_line.py	49.80% <ø> (ø)
...ertpy/tools/_differential_gene_expression/_base.py	32.32% <0.00%> (ø)