docs: update README to align with current API

[michaelbe812]

Summary

• Remove all references to the non-existent rxStateful$ function from README
• Update documentation to emphasize rxRequest as the primary and only API
• Fix incorrect context values and ensure accuracy with current implementation

Problem

As identified in issue #61, the README documentation had drifted from the current API implementation:

1. rxStateful$ function was extensively documented but doesn't exist in the codebase (removed in PR fix: migrate demo app from rxStateful$ to rxRequest #19)
2. Documentation mentioned 'complete' as a context value, but it doesn't exist in the type definitions
3. Examples showed code that wouldn't work with the current library

Changes Made

• ✅ Removed entire rxStateful$ standalone function section
• ✅ Fixed context values (removed non-existent 'complete')
• ✅ Updated all code examples to use rxRequest only
• ✅ Added migration notice about rxStateful$ removal
• ✅ Added comprehensive examples for rxRequest with and without triggers
• ✅ Cleaned up configuration and refresh strategy documentation

Testing

• All README code examples have been verified to compile correctly
• All tests pass: nx test rx-stateful ✅
• All linting passes: nx lint rx-stateful ✅

Fixes #61