VU-1136: Update Skill Builder product docs

[hbanerjee74]

Summary

• Update user guide docs for the current Evals workspace tab instead of the old placeholder wording
• Add end-user help pages for Documents and Optimize Description
• Refresh Dashboard, Settings, Getting Started, and Step 4 links/sidebar entries for the current product surface

Validation

• npx markdownlint docs/user-guide/index.md docs/user-guide/dashboard.md docs/user-guide/settings.md docs/user-guide/test.md docs/user-guide/documents.md docs/user-guide/optimize-description.md docs/user-guide/workflow/step-4-generate-skill.md
• cd docs && npm run build
• repo-map audit: no structural changes detected
• cd app && npx tsc --noEmit
• cargo clippy --manifest-path app/src-tauri/Cargo.toml -- -D warnings

PR phase evidence

• AC gate: pass; all VU-1136 acceptance criteria checked in Linear
• Design conformance: not_applicable; no local design document referenced in issue, comments, attachments, or linked documents
• Promptfoo targeted evals: not_applicable; no changed skills, commands, or eval harness inputs

Fixes VU-1136

[linear-code]

VU-1136 Update Skill Builder product docs to match current product behavior