Update docs for --json flag and Alpine APK install

[vmvarela]

Description

Sprint 4 shipped two user-facing features without corresponding documentation updates:

1. --json flag (JSON output format (--json) #44 / PR feat: add --json output flag #74) — the flag is implemented and tested, but not documented in README.md or the man page (docs/sql-pipe.1.scd).
2. Alpine APK packaging (APK package for Alpine Linux #66 / PR feat: add APK package for Alpine Linux to release workflow #75) — .apk packages are now built in the release workflow, but README.md has no install instructions for Alpine users.

Acceptance Criteria

• README.md: add --json to the Flags table
• README.md: add usage example showing --json output
• README.md: note that --json is mutually exclusive with --delimiter/--tsv/--header
• README.md: add Alpine Linux / APK install section (between RPM and AUR)
• docs/sql-pipe.1.scd: add --json to OPTIONS section
• docs/sql-pipe.1.scd: add example with --json
• docs/sql-pipe.1.scd: document incompatibility with --delimiter/--tsv/--header

Notes

• The --json help text in printUsage() (src/main.zig:99) is already correct — this is docs-only.