Reorganize website navigation

[kwart]

Summary

Reorganizes the Hugo site so the full user guide is the primary docs destination and the old landing page becomes a separate Quickstart.

• Guide (generated from docs/JSignPdf.adoc) moves from /docs/guide/ to /docs/
• Old /docs/ landing page is renamed to /quickstart/ and its menu label changes from Documentation to Quickstart
• Top-nav order is now Guide → Screenshots → Quickstart; homepage CTA and feature links point at the new /docs/ URLs
• prepare.sh emits type: docs in the injected front-matter since there is no longer a parent _index.md cascade to supply it

No content changes — this is purely a URL/navigation reshuffle.

Test plan

• cd website && ./prepare.sh && hugo server — verify /docs/ renders the full guide and /quickstart/ renders the short landing page
• Click Guide, Screenshots, Quickstart in the top nav and confirm each resolves
• From the homepage, click Read the Guide, Start with the GUI →, and CLI reference → and confirm all land on the new /docs/ anchors