Make project guidance and collaboration accessible

[jonashertner]

Outcome

• keeps Method, Sources, Law, About and Contribute reachable from the map at every screen size
• states the whole-cycle ambition and present surface-water scope plainly in all five languages
• adds a dedicated collaboration section for scientific, data, public-sector and legal contributions
• fixes direct section links after data-built figures settle
• adds release contracts for the new navigation and contribution path

Verification

• npm run release
• npm run check:links (81 checked, 0 dead)
• git diff --check
• Chrome: 1440x900, 390x844, 320x568 and 844x390; no horizontal overflow or site-origin console errors