Add AQI documentation for pmsx003 sensor, improve AQI documentation in general

[jasstrong]

Description:

Adds Air Quality Index (AQI) documentation for the pmsx003 particulate matter sensor and enhances the existing hm3301 AQI documentation.

Changes:

• Added AQI configuration option to pmsx003 sensor documentation
• Added detailed "Air Quality Index" section explaining AQI vs CAQI calculation standards
• Enhanced hm3301 AQI documentation with health implication tables
• Documented both AQI (US EPA, 0-500 scale) and CAQI (European, 0-400 scale) standards
• Added configuration examples showing AQI setup with optional filtering
• Included health implication reference tables for both standards

Related issue (if applicable): N/A

Pull request in https://github.com/esphome/esphome with YAML changes (if applicable):

• esphome/esphome#12203

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