chore: remove closed issue 248 documentation files

Removed issue #248 documentation files from docs/issues/:
- 248-docker-ufw-firewall-security-strategy.md (issue spec)
- DRAFT-docker-ufw-firewall-security-strategy.md (draft file)
- manual-tests/248-network-segmentation-test-results.md (manual test results)

Also updated docs/issues/manual-tests/README.md to remove example references.

Issue #248 (Docker/UFW Firewall Security Strategy) has been closed and
merged via PR #249, so these documentation files are no longer needed.

Author: josecelano