Fix typos in multiple documentation files

[gasaicrypto]

This pull request addresses and fixes various typos found in the following documentation files:

• signer-requests.md
• casts.md
• index.md
• migrating.md

The changes improve the overall clarity and correctness of the documentation.

Commits:

• Fixed typos in signer-requests.md, casts.md, index.md, and migrating.md.
• Updated signers.md with minor text adjustments for consistency.

Files affected:

1. docs/reference/warpcast/signer-requests.md
2. docs/developers/guides/writing/casts.md
3. docs/developers/siwf/index.md
4. docs/hubble/migrating.md
5. docs/reference/warpcast/signers.md

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PR-Codex overview

This PR focuses on correcting typos and improving clarity in documentation across various files related to the Farcaster project.

Detailed summary

• Updated "Farcaste" to "Farcaster" in docs/developers/siwf/index.md.
• Corrected "th example" to "the example" in docs/developers/guides/writing/casts.md.
• Fixed "APP_MNENOMIC" to "APP_MNEMONIC" in multiple files.
• Changed "can being" to "can begin" in docs/reference/warpcast/signer-requests.md.
• Updated "signer messages" to "sign messages" in docs/reference/warpcast/signers.md.
• Corrected "locaalhost" to "localhost" in docs/hubble/migrating.md.
• Changed "writing dia" to "writing data" in docs/hubble/migrating.md.

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