Fix DocC build: generate targets separately and merge outputs

[albertodebortoli]

Summary

• Fixes Move DocC tutorials from LucaCore to LucaCLI catalog #66: the second generate-documentation invocation was clearing the output directory before writing, which wiped the LucaCore docs and left only documentation/lucacli/ reachable
• Each target is now built into its own temporary directory (./docs-lucacore and ./docs)
• After both builds complete, LucaCore's target-specific subdirectories (documentation/lucacore/ and data/documentation/lucacore/) are copied into the LucaCLI output — no conflict since each target owns a unique path
• The shared static assets (CSS, JS) come from the LucaCLI build; they are identical to what LucaCore would have produced

Test plan

• Verify CI DocC workflow passes
• Confirm https://lucatools.github.io/Luca/documentation/lucacli/ is reachable after merge
• Confirm https://lucatools.github.io/Luca/documentation/lucacore/ is reachable after merge

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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