docs: improve navigation, cross-references, and guides

[yuecideng]

Description

This PR improves documentation navigation, fixes broken content, and adds new guides to make EmbodiChain docs easier to understand and follow.

P0 — Broken Content Fixed

• Fix dangling sentence in modular_env.rst ("for more randomization events, please refer" → proper cross-reference)
• Clean up resources/task/index.rst broken toctree entries
• Add project overview paragraph to root index.rst
• Add numbered 3-phase learning path with prerequisites to tutorial/index.rst

P1 — Navigation & Cross-references

• Add "See Also" / "Next Steps" sections to: RL tutorial, agents, online data, create_scene, basic_env, RL overview, gym/env overview
• Resolve add_robot duplication: replace guides/add_robot.rst with concise checklist + link to full tutorial

P2 — New Guide Content

• Custom Functors Guide (guides/custom_functors.md): Covers function-style and class-style functor patterns with real code examples for observation, reward, event, and action functors, plus a signature reference table and file placement conventions
• Configuration Guide (guides/configuration.md): Explains @configclass, FunctorCfg hierarchy, JSON config format, string-based function resolution, and SceneEntityCfg usage
• Fix guides/index.rst title underline and add new entries

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Enhancement (non-breaking change which improves an existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (existing functionality will not work without user modification)
• Documentation update

Screenshots

N/A — documentation text changes only.

Checklist

• I have run the black . command to format the code base.
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• Dependencies have been updated, if applicable.

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