Clarifying the documentation for custom env using images as observations

[fracapuano]

SB3 allows to use images as observations for custom environments. However, the current documentation is not 100% clear on the type of observation used. I believe that, especially since images' types are np.uint8, the clarity of the documentation can be enhanced by adding a minimal example, specifying that although the effective type seems to be discrete, images are handled through spaces.Box by SB3.

Description

Minimal change in the custom env documentation guide.

Motivation and Context

• I have raised an issue to propose this change (required for new features and bug fixes)

See above. Mainly enhancing clarity of the documentation to work with custom environment defining images as input observations.

Types of changes

• 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 change)
• Documentation (update in the documentation)

Checklist

• I've read the CONTRIBUTION guide (required)
• I have updated the changelog accordingly (required).
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.
• I have opened an associated PR on the SB3-Contrib repository (if necessary)
• I have opened an associated PR on the RL-Zoo3 repository (if necessary)
• I have reformatted the code using make format (required)
• I have checked the codestyle using make check-codestyle and make lint (required)
• I have ensured make pytest and make type both pass. (required)
• I have checked that the documentation builds using make doc (required)

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[fracapuano]

@araffin perhaps this is very minor, but I still found the interplay of discrete types and Box spaces not super straightforward to understand---I really think this little change could play out to somewhat useful.