forc-doc: Preserve all doc links when merging parallel results

[JoshuaBatty]

Description

The sway-lib-std docs are broken and not showing all of the generated documentaiton. The root index.html was showing only one module while all.html listed only a subset of types. This started happening after the parallel processing was introduced in #7296. This PR fixes this as can be seen in the screenshots below.

Also undoes the CI changes in #7362 & #7365 as they aren't necessary as this was the root cause.

• Root cause: Using BTreeMap::extend to merge per-item results overwrote existing per-category vectors, so only the last chunk from parallel processing survived.
• Fix: replace .extend merges with per-key append-and-dedup loops.

Before:

After:

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[codspeed-hq]

CodSpeed Performance Report

Merging #7379 will not alter performance

_{Comparing josh/doc-lib-std (fa86425) with master (3210f94)}

Summary

✅ 25 untouched benchmarks