Update README to accurately document PreMailer API

[devin-ai-integration]

Updated README to accurately document PreMailer API

This PR updates the README to ensure it accurately reflects the library's main API:

• Added documentation for missing parameters: preserveMediaQueries, preserveEntities, and customFormatter
• Enhanced examples showing how to use the instance API with all options
• Added a dedicated section for the AddAnalyticsTags method
• Added a section for the Document property for custom DOM processing
• Improved the documentation for media query preservation

Link to Devin run: https://app.devin.ai/sessions/ca2c35c52aad44439841b815f5a6d70c
Requested by: [email protected]

[devin-ai-integration]

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