chore: Update SDK example for app and EVM proofs

[ytham]

• Splits Book SDK section into two subsections for app and EVM proofs.
• Separates sdk example sdk.rs into two files: sdk_app.rs, sdk_evm.rs
  • NOTE: there is a fair amount of reused code; this was done intentionally for the sake of making the examples easier to read in isolation, as well as to keep the imports in the book a bit cleaner. I can create some common functions instead if it's preferred.

[jonathanpwang]

I would prefer if the sdk md stays in one file, we can make the EVM Proof part a larger heading and explain that the generate_app_proof does not need to be called.

The sdk_app.rs, sdk_evm.rs split makes sense.

[jonathanpwang]

LGTM