docs: add cached CI pipelines guide and document finalize/skip commands

[gregberge]

Description

Use-case-driven documentation for parallel finalize mode, motivated by setting up Argos in a Turborepo monorepo where the cache decides which test suites run.

• New guide: Cached CI pipelines (Turborepo, Nx) — the full recipe: aggregate packages into one parallel build (finalize mode), mark PR builds as subset, force complete baseline-eligible builds on main with a cache-busting env key, and keep a required check green with argos finalize --skip-if-empty. Includes a complete GitHub Actions example, the Turborepo strict-env-mode passthrough gotcha, and an FAQ.
• Rewrote Parallel testing (sharding) around choosing between the two modes: count mode (fixed, guaranteed uploads) vs finalize mode (dynamic shard counts, conditional uploads, aggregated suites), with a warning about count-mode builds hanging when uploads are skipped, and a troubleshooting section. Also fixes grammar and the dangling env: block in the finalize example.
• CLI reference: added finalize and skip command sections — finalize was previously only visible inside a YAML example, and skip only in the skipping-a-build guide.
• Cross-links: subset builds, skipping a build, and monorepos setup now point to the new guide.

Further comments

--skip-if-empty ships in argos-ci/argos-javascript#341 — this PR should be published after that release.

The turbo env-precedence claim in the guide (a hashed env declaration invalidates the cache even when a passthrough wildcard also matches the variable) was verified experimentally against turbo 2.9.18.

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