Unified Compiler documentation

[isaacdevlugt]

Context:

• Some parts of the xDSL API can start to be more exposed, which requires documentation
• Our transforms API documentation is scattered all over the place. We want a singular source of truth that hosts applicable transforms no matter where they live under the hood.

Description of the Change:

This PR will aim to do two things:

• Ensure that applicable functions in the unified compiler work are properly documented in PennyLane (there will be an adjacent PR opened in Catalyst as well)
• Create (or revamp) a documentation page that will host every optimization transform available in our stack (including Catalyst). "Optimization transforms" are transforms that make the circuit more efficient given constraints, but doesn’t change the user output.

Benefits: 🤑

Possible Drawbacks: 🤷

Related SC Epic: https://app.shortcut.com/xanaduai/epic/92676

[github-actions]

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