docs: improve Ajv instance README guidance

[vacekj]

Summary

• clarify that validator.ajv must be configured before calling validate()
• add a concrete ajv-formats example for schemas using format: "email"
• update surrounding README examples and migration notes to point readers to the exposed Ajv instance

Closes #113

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Note

Low Risk
Documentation-only updates; no runtime code changes, with minimal risk beyond potentially confusing guidance if incorrect.

Overview
Updates README.md to clarify Ajv configuration order: create a Validator, configure validator.ajv (formats/plugins/custom keywords), then call validate() so schemas compile with the configured Ajv instance.

Refreshes examples to instantiate validator explicitly (instead of destructuring from new Validator()), adds a concrete ajv-formats/format: "email" walkthrough, and adjusts the v2→v3 migration note and Ajv option link to point readers at the new guidance.

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