Use combined documentation to build all targets into a single site

[albertodebortoli]

Summary

• Replaces the manual two-step build + cp -r merge from Fix DocC build: generate targets separately and merge outputs #68 with a single --enable-experimental-combined-documentation invocation
• This flag (swift-docc-plugin 1.4+) builds all package targets, passes --dependency flags between them so cross-target links resolve, and merges everything into one combined archive
• Fixes the "page not found" issue on documentation/lucacore/ — the manual merge was copying HTML but missing the backing JSON data files that DocC's client-side router needs to render content

What changed

The DocC CI step is now a single command:

swift package generate-documentation \
  --enable-experimental-combined-documentation \
  --disable-indexing \
  --output-path ./docs \
  --transform-for-static-hosting \
  --hosting-base-path Luca

Test plan

• Verify CI DocC workflow passes
• Confirm https://luca.tools/Luca/documentation/lucacli/ loads with tutorials
• Confirm https://luca.tools/Luca/documentation/lucacore/ loads with API reference
• Verify cross-target links work (e.g. LucaCLI referencing LucaCore types)

[albertodebortoli]

Finally. This worked!