Add usage docs

[amitschang]

Working through resolving https://github.com/ssec-jhu/dplutils/issues/80. This PR covers the major sections. I will save extensions and observers for another PR to make this more digestible.

[codecov[bot]]

Codecov Report

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

Project coverage is 100.00%. Comparing base (2f9dfd7) to head (4ddbd1e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #81 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 11 11 Lines 570 570 ========================================= Hits 570 570 ``` | [Flag](https://app.codecov.io/gh/ssec-jhu/dplutils/pull/81/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ssec-jhu) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/ssec-jhu/dplutils/pull/81/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ssec-jhu) | `100.00% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ssec-jhu#carryforward-flags-in-the-pull-request-comment) to find out more.

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[xiangchenjhu]

Thanks!