Fix Learn Exercise Submissions

[ericbrown99]

What changed? Why?

• Removed the old rainbowkit button from the Learn section and replaced it with onchiankit.
• Changed learn .md files to .mdx for proper component rendering in Vocs
• Upgraded the implementation of wagmi in the learn CafeUnitTest component to use wagmi v2

*Notes to reviewers
How has it been tested?
Ran locally

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Name	Status	Preview	Comments	Updated (UTC)
base-web	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Mar 7, 2025 9:02pm
web-base-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Mar 7, 2025 9:02pm

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1