Remove built-in HTML reporter, recommend @testomatio/reporter

[DenysKuchma]

Remove the built-in htmlReporter plugin to consolidate HTML reporting functionality into the external @testomatio/reporter package. This change reduces maintenance burden and provides users with a more feature-rich reporter that's actively developed separately. #5406

The built-in htmlReporter was duplicating functionality already available in @testomatio/reporter, which offers:

• Modern, responsive interface with better design
• Active development and maintenance
• Optional integration with Testomat.io cloud
• Support for S3 uploads and custom report locations
• Consistent API across all testomat.io products

Changes:

• Delete lib/plugin/htmlReporter.js and all related tests
• Replace documentation in docs/reports.md and README.md
• Remove htmlReporter from default config template (lib/command/init.js)
• Delete old htmlReporter screenshots

Docs: https://docs.testomat.io/test-reporting/pipes/html/

Migration for users:

# Install
npm install @testomatio/reporter --save-dev

# Configure
plugins: { testomatio: { enabled: true } }

# Run
TESTOMATIO_HTML_REPORT_SAVE=1 npx codeceptjs run