Add docs for BioImage.IO: finding & sharing models

[qin-yu]

53aa2dc adds blurbs for finding and exporting Cellpose models on BioImage.IO, extending #988. I aimed to make it clear and informative without copying docstrings. A checklist is included along with usage in --help and link to an example in the docstring for easy start.

I noticed some inconsistencies in doc file line widths, and I kept auto-formatting to improve readability in 3445bc7

[codecov]

Codecov Report

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

Project coverage is 53.17%. Comparing base (3445bc7) to head (a70f773).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
cellpose/export.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1011      +/-   ##
==========================================
- Coverage   53.64%   53.17%   -0.48%     
==========================================
  Files          18       18              
  Lines        4138     4139       +1     
==========================================
- Hits         2220     2201      -19     
- Misses       1918     1938      +20     

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[qin-yu]

I just double-checked the definition of a "blurb" : ) The exporting section goes a bit beyond the usual 200-word limit. Let me know if you'd prefer adjustments to the length or formatting.

[carsen-stringer]

hah no longer is always better thank you so much!