docs: update Base Goerli references to Sepolia and fix outdated links

[Himess]

What changed? Why?
This PR updates all remaining references from Base Goerli to Base Sepolia across the documentation and fixes several outdated or broken links.
All content now aligns with the current Base Sepolia testnet infrastructure and the latest official resource URLs.

Changes

• Replaced all Goerli and GOERLI references with Sepolia and SEPOLIA in relevant guides.
• Updated Base Goerli → Base Sepolia examples in code snippets, .env variables, and faucet instructions.
• Replaced deprecated Mirror blog links (base.mirror.xyz/...) with the new Base Blog URLs (blog.base.org/...).
• Fixed incorrect or outdated documentation links:
  • https://docs.xmtp.org/agents → https://docs.xmtp.org/agents/get-started/build-an-agent
  • Old Metamask testnet config link → updated support article
  • Oracles docs updated to /base-chain/tools/oracles
  • Outdated OpenZeppelin v2.x docs → v5.x canonical URLs
• Fixed a malformed Study Hall Creative link (removed stray %20).
• Updated example repository references (e.g., MiniKit demo path under /mini-apps/templates/).

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Context

These changes ensure all learning materials, code samples, and resource links reflect the migration from Goerli → Sepolia as outlined.

Notes

All required Goerli → Sepolia conversions have been applied consistently across the entire /docs directory, including environment variables, faucet URLs, and testnet deployment guides.
Notes to reviewers

How has it been tested?

[cb-heimdall]

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