Add content to README; documentation fixups

[rkingsbury]

Summary

Makes README more descriptive of `maggma''s purpose and function. Closes #886

See a preview of the README on my fork here: https://github.com/rkingsbury/maggma/tree/readme

[rkingsbury]

@Andrew-S-Rosen any suggested revision are welcome!

@munrojm does it make sense to add a paragraph about what the API piece of maggma does? If so, could you post some text here and I can integrate it?

Looking at the docs, I realize that there is currently nothing in there about the API components at all; that's probably something we should add at some point (separate issue though)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.77%. Comparing base (84f864e) to head (6d58e39). Report is 11 commits behind head on main.

:exclamation: Current head 6d58e39 differs from pull request most recent head 84d0749

Please upload reports for the commit 84d0749 to get more accurate results.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #969 +/- ## ======================================= Coverage 88.76% 88.77% ======================================= Files 46 46 Lines 3979 3981 +2 ======================================= + Hits 3532 3534 +2 Misses 447 447 ```

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[Andrew-S-Rosen]

Looks great! Did we want to add a minimal example so people visually know what we are talking about? Something like

my_data = {"a": 1, "b": 2}
store = MongoStore( database="my_db_name", collection_name="my_collection_name", username="my_username", password="my_password", host="my_hostname", port=27017, ) 
with store:
        store.update(my_data)

[Andrew-S-Rosen]

Looks good to me!

[Andrew-S-Rosen]

That example is hilarious!

[rkingsbury]

Now that I realize we don't have any documentation of the API components, I'm going to remove the placeholder from the README and merge this. I've opened #972 as a placeholder to remind us that this documentation should be added at some point.

[rkingsbury]

There is an open_data test failure that snuck in on the last dependency upgrade, which I can't reproduce locally. I'm merging this b/c it's unrelated and will see if re-triggering the auto dependency upgrade can resolve.