Basic OOC guide

[ivirshup]

This PR creates a page that goes over the various ways to work with AnnData objects in storage.

• Closes "How to" for read_elem, write_elem #1159
• [ ] Tests added
• Release note added (or unnecessary)

TODO:

• Combining section
• Dense dask array examples
  • IO for sparse dask arrays
  • Conversion b/w sparse and dense dask arrays
• Pick an example dataset to use
• Make code chunks work
• Move "Concatenation" doc into guides?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.12%. Comparing base (a478647) to head (f88f7bd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1163      +/-   ##
==========================================
- Coverage   86.23%   84.12%   -2.12%     
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  Files          35       35              
  Lines        5592     5592              
==========================================
- Hits         4822     4704     -118     
- Misses        770      888     +118     

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