Docs updates: add FileStore and misc edits

[rkingsbury]

Documentation updates for maggma. The primary purpose of this PR is to add thorough documntation of the recently-merged FileStore (see #619 ).

I also took the opportunity to make some edits, including:

• Correcting inaccurate language about python 3.6 support which was dropped
• Re-ordering some documentation sections based on what I think a newcomer most needs to know
• Wordsmithing and typos

I have inspected the changes using mkdocs serve and all seems to compile cleanly, but let me know if you spot any issues!

[codecov]

Codecov Report

Merging #668 (a1b5222) into main (e84dc1d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #668   +/-   ##
=======================================
  Coverage   89.43%   89.43%           
=======================================
  Files          42       42           
  Lines        3048     3048           
=======================================
  Hits         2726     2726           
  Misses        322      322           

Impacted Files	Coverage Δ
src/maggma/stores/file_store.py	98.02% <ø> (ø)

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[munrojm]

The new documentation looks fantastic @rkingsbury! Really appreciate the effort in both adding to and cleaning up what was there. I will go ahead and merge these changes.