Gymnasium migration: docs requirements and text refactoring

[kingsleynweye]

Description

Relaxed Sphinx documentation build requirements and updated mentions of "OpenAI Gym" in docs to "Farama Foundation Gymnasium".

Issue

104

Changes

See description.

Screenshots

NIL

Checklist

• [x] I have tested the changes locally and they work as intended.
• [x] I have updated the documentation, if applicable.
• [x] I have added new tests, if applicable.
• [x] I have added any required dependencies to the requirements.txt file, if applicable.
• [x] I have followed the project's code style and conventions.

Additional notes

NIL