docs: complete Middleware package docs for v3.0

[armanist]

Summary

• updated v3.0/packages/middleware/overview.md
• updated v3.0/packages/middleware/architecture.md
• updated v3.0/packages/middleware/contracts.md
• updated v3.0/packages/middleware/usage.md

Coverage

• completed the Middleware package revisit for route and group execution order, pipeline lifecycle, and constructor expectations

Notable contracts

• documented that group middleware runs before route-specific middleware
• documented that middleware instances are created when their turn in the pipeline is reached
• clarified that custom constructors receive the current Request instance