docs: add Spanish onboarding and clear language fallback

[rmcdaniel]

Change

Delivers the Spanish onboarding increment of #169, with 11 guides covering deployment choice, installation, a first completed workflow in PHP/Python/Rust, embedded authoring, execution concepts, determinism, idempotency and recovery. Home, navigation, sidebar categories and search messages are translated. Code, API identifiers, artifact placeholders and heading anchors stay aligned with English.

Untranslated documentation clearly identifies its English content, provides a direct language link and marks the body language correctly. Its canonical URL points to English while preserving existing explicit canonical metadata. English remains the default and Ukrainian remains available.

The existing documentation build notices English source changes needing Spanish review and requires executable code blocks to match. Documentation maintainers own the recorded review and update procedure. Prose changes produce a review notice without becoming a snapshot test.

A small pinned Docusaurus 3.10.2 utility patch resolves relative links between translated and fallback files using the actual document map. Missing-link checks remain strict. README documents its upgrade/removal check.

Verification

• Fresh Node 24 dependency installation applies the patch successfully.
• Full English, Ukrainian and Spanish build passes with strict link and anchor checks.
• Code blocks match English in all 11 Spanish guides.
• Seven resolver cases pass, covering both fallback directions, custom slugs, missing targets and version boundaries.
• A source prose edit emits the review notice without blocking the build. A changed executable command fails.
• Chromium checks pass translated routes, canonical/hreflang metadata, all three quickstart tabs, English fallback navigation, accented Spanish search, stable anchors, English/Ukrainian routes and desktop/mobile language switching, with no runtime exceptions.
• Final source 18c37a6a7082499ddcbfc4d331ea0d5b66e93eb1 passes documentation qualification, public-boundary and protocol checks.

Spanish wording and technical meaning were reviewed against the English source, including runtime ownership, at-least-once activity execution, committed history, stale reports, retry limits and terminal workflow behavior.

Remaining roadmap

#169 stays open for the subsequent locales and audience/review decisions. Reference pages outside this Spanish increment, blog articles and 1.x documentation remain in English. Merged main a620f9a9fabe57480ed24631587a69245e56f757 passes all checks and Pages deployment. Live Chromium repeats the complete route, navigation, search, metadata and desktop/mobile checks successfully. The merged branch is deleted.