Add comprehensive Build and Packaging documentation

- New docs/build-and-packaging.md covering:
  - Build artifacts (TXZ packages and PLG descriptors)
  - Package structure and creation scripts
  - Version management and build numbers
  - PLG entity system for maintainability
  - SHA256 vs MD5 integrity verification
  - Version upgrade detection logic
  - CI/CD pipeline with GitHub Actions
  - Multi-environment deployments (PR/staging/production)
  - PR preview build lifecycle
  - Artifact cleanup and retention policies
  - Service control scripts (rc.d)
  - Web components for legacy integration

- Updated docs/plg-file.md:
  - Added SHA256 as preferred verification method
  - Updated integrity checksum best practices
  - Fixed next steps links

- Updated index.md navigation

Source: deepwiki.com/unraid/api/4.1-plugin-build-and-packaging

Author: mstrhakr