Restructuring: "Build Your Own Chain" content

[Cordtus]

Summary:

Consolidated and reorganized the chain building documentation to reduce duplication and improve navigation flow, while maintaining 100% of the existing coverage.

Key Changes:

• Merged content: Moved relevant building-your-chain-guide.mdx content into overview.mdx (Introduction page) for single entry point
• Removed token-configuration-guide.mdx (content already in pre-genesis-and-genesis-setup.mdx) and chain-customization-checklist from navigation
• Improved organization: Reorganized Introduction page with clearer flow: Where To Start → Main Guides → Resources → Advanced → Testing
• Enhanced navigation: Moved Configuration Reference below Additional Configuration group for logical ordering
• Better formatting:
  • Made large codeblocks expandable with Accordions in additional configuration docs
  • Wrapped parameter CardGroups in Accordions for cleaner presentation
  • Removed card icons for simpler appearance
• Cross-references: Added links between related documentation sections