docs: fix config environment variables in README

[akbad]

Fixes multiple environment variable documentation issues:

• Update FastMCP variable names to match 2.7.x API (FASTMCP_SERVER_* prefix)
• Add missing QDRANT_SEARCH_LIMIT and QDRANT_READ_ONLY documentation
• Adjust claim that command-line arguments are not supported to include the still-supported --transport flag
• Reorganize "Environment Variables" section slightly to clearly present important information upfront

Note: overlaps with #105 but more comprehensively resolves #103, handling all FastMCP variables and adding missing Qdrant variables.

[tbung]

Thank you, I'll merge this to "stop the bleeding" even though this'll change again once I get around to bump the fastmcp version in #125