refactor(ci): drop tar shuffling, each runner does its own checkout + cache

Replaces the workspace-tar approach (which had --exclude pitfalls and
included fork-only files that shouldn't affect cache) with the upstream
pattern: each matrix runner does its own actions/checkout, restores
deps from actions/cache by content hash, and computes its Hugo output
cache key from Hugo-relevant files only.

Cache key inputs are now strictly Hugo-relevant:
  - content/<version>/** (per-version) or content/** without version dirs (latest)
  - layouts/**, assets/**, data/**, static/**, i18n/**
  - config.toml, postcss.config.js, tailwind.config.js, package-lock.json
  - version lists (kubernetes-versions, rs-versions, ...)
  - .github/workflows/airgap-build.yml (so workflow logic changes invalidate)

Fork-only files (helm/, Dockerfile, airgap-multibuild.sh, BUILD.md, ...)
no longer enter any cache key. Editing them is a cache HIT across the
board.

Caches per runner (workspace-relative paths, no sudo needed):
  - node_modules         keyed on package.json + package-lock.json
  - ./venv               keyed on requirements.txt
  - examples             keyed on Makefile + build/** + data/components_local/index.json
  - output (Hugo result) keyed on the per-build hash described above

setup is now lean: just checkout, discover versions, emit matrix JSON.
No tar, no patches, no excludes. Setup ~30 sec instead of ~5 min.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: EliShteinman