Update homepage text and add logo

[elliottower]

Description

Text formatting changes from #471, going back to the text used in 0.28.1 which was more concise and cleaner for the homepage.

I also made the second sentence in the description not bold so it's easier to read at a glance and see that it's a maintained fork of OpenAI Gym. We can change that back if we want though.

Before: "The Gymnasium interface is simple, pythonic, and capable of representing general RL problems, and has a compatibility wrapper for old Gym environments:"

After: "Gymnasium is a maintained fork of OpenAI’s Gym library. The Gymnasium interface is simple, pythonic, and capable of representing general RL problems, and has a compatibility wrapper for old Gym environments:"

Edit: I've also changed the homepage styling to use the nice graphics we use in the GitHub and do a separate header and title (one long title is very ugly and I've had multiple people tell me it should be changed)

Edit2: Changed the word 'set' to collection as that was the previously used wording, and makes the second line look better (set makes all the text fall only on the top line).

Edit3: Changed wording according to internal discussions

3:

2:

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Fixes # (issue)

Type of change

Please delete options that are not relevant.

• 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

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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