docs: add development setup guide

[beanscg]

Fixes #58.

This adds a dedicated development setup guide and links it from the README.

The new guide covers:

• Go version from go.mod
• Linux/BPF prerequisites, including BPF LSM and IMA checks
• ig CLI installation and verification
• Kernel boot-parameter guidance for enabling BPF LSM
• Linux VM options for non-Linux hosts
• Build order: make build-gadgets then make build-app, plus make build-all
• Why sudo is needed for BPF/LSM development commands
• Audit vs enforce mode examples
• Docker-based test workflow
• Validation commands and common troubleshooting cases

Validation:

• git diff --check
• Direct content check for the new development guide and README link
• go test ./... was attempted on macOS. Several packages passed, but the full suite cannot complete on this Darwin host because Inspektor Gadget/BPF dependencies are Linux-specific:
  • build constraints exclude all Go files in github.com/inspektor-gadget/inspektor-gadget/pkg/utils/host
  • undefined: unix.CLOCK_BOOTTIME