Update docs with EpisodeData info

[enerrio]

Description

This PR adds a section to the Dataset Standards documentation page describing the EpisodeData data structure and the fields it contains. Also includes a small snippet of sampling episodes from a Minari dataset.

Type of change

• This change requires a documentation update

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have run pytest -v and no errors are present.
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I solved any possible warnings that pytest -v has generated that are related to my code to the best of my knowledge.
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[balisujohn]

One change, otherwise, looks good!

[balisujohn]

Observations and actions are not necessary represented as a single np.ndarray; it's worth looking at episodes from the various dummy envs used in test_dataset_creation.py, to get an idea of what is possible for observations and action types.

[enerrio]

@balisujohn I updated the data types with the test spaces I saw in the test_dataset_creation.py and common.py files. Let me know what you think.

[balisujohn]

I found the types tuple, list and dict, and numpy.ndarray from when I added at line 74 of test_dataset_creation.py:

    print(type(dataset[0].observations))

It might be fair to say List[str] though if so, I'd confirm that string spaces are the only context where lists occur.

[enerrio]

I see, I updated data types. When it comes to list I only saw data type List[str] in datasets created in test_dataset_creation.py but left it as list in case there's some other dataset type I'm missing.

[balisujohn]

LGTM :^)