docs: add index page for learn section

[samricotta]

Description

Updates the documentation with an overview page highlighting what is in the section. Closes issue in the docs repo.

Closes: cosmos/cosmos-sdk-docs#96

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[coderabbitai]

Walkthrough

The markdown document has been expanded with structured educational content about the Cosmos SDK. It now includes an introduction, as well as sections catering to beginners and advanced users, providing a comprehensive learning path from foundational knowledge to more complex topics.

Changes

File Path	Change Summary
docs/learn/learn.md	Added sections: "Introduction", "Beginner", "Advanced", with detailed subtopics related to Cosmos SDK learning progression.

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