Add GPS documentation page

[mairas]

Summary

• New user-guide/gps.md page documenting gpsd in HaLOS marine variants
• Covers default configuration, Signal K integration, command-line tools, USB auto-detection, and HALPI2 GNSS HAT reference

Test plan

• Build docs locally (mkdocs serve) and verify page renders correctly
• Verify navigation entry appears under User Guide after "Marine Apps"
• Check cross-link to HALPI2 docs resolves

🤖 Generated with Claude Code

[mairas]

Review

Scope: New user-guide/gps.md page and mkdocs.yml nav entry.

Content review:

• Correctly describes gpsd's role and default configuration (DEVICES="", USBAUTO=true)
• Good distinction between generic HaLOS Marine (USB auto-detect only) and HALPI2 (UART0 + u-blox auto-config)
• Signal K integration correctly noted as automatic
• Command-line examples (gpsmon, gpspipe -r, gpspipe -w) are practical and useful
• The note about gpsd using JSON protocol (not raw NMEA) is helpful for users expecting NMEA sentences

Nav placement: After "Marine Apps" and before "App Data and Migration" — logical grouping.

Cross-link: References HALPI2 GNSS docs at docs.hatlabs.fi/halpi2/user-guide/interfaces/#gnss-gps — this anchor will exist once hatlabs/halpi2#16 is merged. If that PR is merged first, the link will resolve correctly. If this PR is merged first, there will be a brief broken link.

No issues found. Approved — can merge once CI passes.