DOC: EX01 (part of ExtensionArray)
- [x] Towards #37875

`ExtensionArray` is kind of tricky, I don't know what kind of example should we put here. Currently I'm providing some (not directly runnable) example implementation for each method. Alternatively we can provide a fully implemented example in `ExtensionArray`, and link to that example in each method. Please let me know if I should make any modification.

im skeptical of it being worthwhile to implement subclasses in docstrings like this yeah might be better to just instantiate examples of EAs, and show an example of these methods being used on them?

like
```python
In [1]: pd.array([1, 2, 3]).nbytes
Out[1]: 27
``` Sure, I'll re-do this PR.