Update Taxi environment documentation to clarify starting state definition

[britojr]

Description

Update the documentation to specify that the starting state is sampled uniformly over 300 states, thus excluding the 100 unreachable states and the 100 states where the passenger is already in the taxi.

Fixes #1115

Type of change

Please delete options that are not relevant.

• Documentation only change (no code changed)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Screenshots

Please attach before and after screenshots of the change if applicable.
N/A

Checklist:

• I have run the pre-commit checks with pre-commit run --all-files (see CONTRIBUTING.md instructions to set it up)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• 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

[britojr]

Hi @Kallinteris-Andreas, here is the PR for issue #1115

[pseudo-rnd-thoughts]

I've merged as the errors seem unrelated to the PR, will investigate in a separate PR