Update Circom optimization flag documentation

Issue Description

The documentation in docs/zk-email-verifier/usage-guide.md contains outdated information about Circom's default optimization flags.

Current Documentation (Line 68)

Additionally, we generally recommend using the --O0 flag for optimization during compilation for beginners. However, if you're more experienced with Circom, feel free to use the --O1 flag instead. It's important to avoid using the --O2 flag as that is the default setting and it may lead to the deletion of additional constraints.

Correct Information

• --O1 is the current default (since circom 2.2.0)
• Before version 2.2.0, --O2 was the default

Suggested Fix

The documentation should be updated to reflect that --O1 is now the default optimization level in Circom 2.2.0+, and adjust the recommendations accordingly.

Location

File: docs/zk-email-verifier/usage-guide.md
Lines: 68-69
Section: Step 3: Compile your circuit

Open docs/zk-email-verifier/usage-guide.md and review lines 68-69 in “Step 3: Compile your circuit.” Update the optimization guidance to state that Circom 2.2.0+ defaults to --O1 and that versions before 2.2.0 defaulted to --O2. Done means the documented defaults and recommendations match those version differences.