docs: flesh out rendered docs homepage

[professor314]

Summary

Improves the documentation homepage by adding several new sections that help newcomers understand the library at a glance.

Changes

• Why distreqx? section summarizing key advantages (pure JAX, equinox-based, actively maintained, strict design patterns, no batch dimension)
• Features at a Glance tables listing all available distributions and bijectors with their class names
• Additional quick examples for bijector usage, transformed distributions, and JAX transformations (vmap, grad)
• Renamed Quick example to Quick Examples to reflect multiple examples

Motivation

Addresses #22. New users benefit from seeing the full scope of available distributions/bijectors upfront, understanding why to choose distreqx over alternatives, and seeing examples beyond just distributions.

Testing

Documentation-only change. Verified the markdown renders correctly and all references are valid.

[lockwo]

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