docs: add documentation for new differential abundance method

[lordy5]

Added a page in the user guide to explain the new differential abundance method, similar to the existing page for differential expression.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.08%. Comparing base (6a78230) to head (ebdf0f8).
⚠️ Report is 1 commits behind head on main.

❌ Your project check has failed because the head coverage (71.08%) is below the target coverage (75.00%). You can increase the head coverage or adjust the target coverage.

❗ There is a different number of reports uploaded between BASE (6a78230) and HEAD (ebdf0f8). Click for more details.

HEAD has 93 uploads less than BASE

Flag	BASE (6a78230)	HEAD (ebdf0f8)
	96	3

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3727       +/-   ##
===========================================
- Coverage   86.75%   71.08%   -15.68%     
===========================================
  Files         229      229               
  Lines       22622    22622               
===========================================
- Hits        19626    16080     -3546     
- Misses       2996     6542     +3546     

see 82 files with indirect coverage changes

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