docs: add audience notes to get-started pages

[devacc8]

Summary

Added "Who is this for?" notes at the top of key get-started pages to help readers quickly identify if the content matches their needs and experience level.

Changes

Added audience notes to 6 pages in the get-started section:

Page	Target Audience
deploy-smart-contracts.mdx	Developers with basic Solidity knowledge
build-app.mdx	Frontend developers familiar with React/Next.js
launch-token.mdx	Creators and developers launching tokens
get-funded.mdx	Builders seeking funding
ai-prompting.mdx	Developers using AI tools
base-mentorship-program.mdx	Builders seeking mentorship

Example

<Note>
**Who is this for?** Developers with basic Solidity knowledge who want to deploy their first smart contract on Base.
</Note>

Fixes #806

[cb-heimdall]

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