Add documentation and colorbar settings

[ghiggi]

Prework

• [x] I understand and agree to this repository's code of conduct.
• [x] I understand and agree to this repository's contributing guidelines.
• [x] I have already submitted an issue or discussion thread to discuss my idea with the maintainers.

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [x] Feature
• [x] Documentation
• [x] Tutorial
• [ ] Code style update
• [ ] Refactor
• [ ] Build-related changes
• [ ] Other, please describe:

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

If yes, please describe the impact and communicate accordingly:

The PR fulfills these requirements:

• [ ] It's submitted to the branch named as follow :
  • Fix a bug: bugfix-<some_key>-<word>
  • Improve the doc: doc-<some_key>-<word>
  • Improve a tutorial tutorial-<some_key>-<word>
  • Add a new feature: feature-<some_key>-<word>
  • Refactor some code: refactor-<some_key>-<word>
  • Optimize some code: optimize-<some_key>-<word>
• [ ] When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• [ ] Don't forget to link PR to issue if you are solving one.
• [ ] All tests are passing.
• [ ] New/updated tests are included

Summary

Add GPM-API documentation and colorbar configurations YAML files

[codecov[bot]]

Codecov Report

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

Project coverage is 87.76%. Comparing base (cf70151) to head (a9efab1).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #48 +/- ## ======================================= Coverage 87.75% 87.76% ======================================= Files 109 110 +1 Lines 10459 10459 ======================================= + Hits 9178 9179 +1 + Misses 1281 1280 -1 ```

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[coveralls]

coverage: 87.762% (+0.01%) from 87.752% when pulling a9efab1a8c0d39f403aa65f1f7d5478ad7337c68 on add-imerg-colorbar-settings into cf70151b3cb0ce2ad941c57969addec45a110534 on main.