Created READMEs for observers and checkpointing

[codecov-commenter]

Codecov Report

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

Comparison is base (bd19c6a) 91.53% compared to head (d1ac035) 91.51%. Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #463 +/- ## ========================================== - Coverage 91.53% 91.51% -0.02% ========================================== Files 162 162 Lines 21738 21717 -21 ========================================== - Hits 19897 19875 -22 - Misses 1841 1842 +1 ```

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