Update Context7 MCP documentation to use standard Authorization header

[oso95]

Summary

• Updates the MCP add command documentation to use the standard Authorization: Bearer header format
• Changes from custom CONTEXT7_API_KEY header to OAuth 2.0 standard format

Changes

• Modified the claude mcp add command example in README
• Updated header format from "CONTEXT7_API_KEY: YOUR_API_KEY" to "Authorization: Bearer YOUR_CONTEXT7_API_KEY"

Rationale

The Authorization: Bearer format is the industry standard for API authentication and provides:

• Better compatibility with existing tools and libraries
• Immediate recognition by developers familiar with OAuth 2.0
• Consistency with HTTP authorization best practices
• Clear indication that this is an authentication header