docs(python): add documentation for local usage

[jayceslesar]

Which issue does this PR close?

Closes #6471

Rationale for this change

Adds some documentation on local usage to go over the root concept, as well as show some basic examples of reading and writing.

I think it makes sense to explicitly mark these docs as local usage as I was playing around with S3, and noticed that we do not need to specify a root like we do for local usage, which is why I added the documentation about the root argument/concept. I am happy to write more docs for S3 usage in the future!

I believe these docs can be slightly cleaned up if/when #6479 is implemented as it seems clunky to have to explicitly get the Path object as a string in the example code.

What changes are included in this PR?

Just documentation changes.

Are there any user-facing changes?

No code changes, but documentation is user-facing :)

[erickguan]

You wrote a great guide.

I recommend putting this under either Examples or having a new section called "Tutorials".

I know the index page can get some more love but being concise is nice.

[erickguan]

Great, thanks!

[jayceslesar]

@erickguan happy to add more detailed examples to that section in the future! (S3, google drive, local ect)

[erickguan]

Please do! Happy to discuss this with you if you are not sure where to organize the documentation.