updated documentaion.

[jmmshn]

Added docs for S3Store.

• explanation of how the stores work
• configuration
• parallelism.

[codecov]

Codecov Report

Merging #419 (6a9b7f5) into main (af94b29) will decrease coverage by 0.22%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #419      +/-   ##
==========================================
- Coverage   86.96%   86.74%   -0.23%     
==========================================
  Files          28       28              
  Lines        2225     2225              
==========================================
- Hits         1935     1930       -5     
- Misses        290      295       +5     

Impacted Files	Coverage Δ
src/maggma/stores/aws.py	85.22% <ø> (ø)
src/maggma/stores/mongolike.py	82.12% <0.00%> (-1.91%)	⬇️

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