docs: PR-C — rebrand CLI references + new shim shape (#198, #203)

[ChrisonSimtian]

Closes #198 and completes PR-C of #203.

Scope

Two related sweeps bundled into one PR:

1. CLI/brand rebrand (closes Docs CLI cleanup: replace stale nuke references with fallout across docs/ #198) — replace stale nuke CLI command examples with fallout across docs/, switch .nuke directory-marker references to .fallout, rebrand "NUKE provides/comes/has/integrates" phrasing to "Fallout …" where it describes the current product, update namespace examples (Nuke.Common.Tools.* → Fallout.Common.Tools.*, Nuke.GlobalTool → Fallout.GlobalTool), and update MSBuild property names (NukeRootDirectory → FalloutRootDirectory, with the legacy names called out as still-honoured).
2. New shim shape (PR-C of Deprecate per-repo bootstrappers; make Fallout.GlobalTool the canonical entry point #203) — update CONTRIBUTING.md, CLAUDE.md, docs/01-getting-started/{02-setup,03-execution}.md, and docs/05-cicd/github-actions.md to describe the thin shims + local-tool-manifest pattern PR-B introduced: drop build.cmd from examples, document the new 3-step GitHub Actions shape, describe .config/dotnet-tools.json.

What's NOT touched

• docs/rebrand-plan.md — intentionally NUKE-named throughout (it IS the rebrand plan).
• docs/migration/from-nuke.md — migration framing needs both names.
• Historical references like "upstream NUKE issue #822" and "the original NUKE key was matkoch-owned" — they cite the upstream project and need to stay accurate.
• CHANGELOG.md and package READMEs that reference the historical NUKE project for migration context.

Files changed

22 docs + CONTRIBUTING.md + CLAUDE.md (23 files total).

Most edits are mechanical text substitutions. Substantive rewrites:

• docs/01-getting-started/02-setup.md — new file-structure example (drops build.cmd, adds .config/dotnet-tools.json and .fallout/), new note explaining the thin-shim role, MSBuild property names updated.
• docs/01-getting-started/03-execution.md — thin-shim explanation in the Global Tool / Windows / Linux tab block; all nuke <target> / nuke --help / nuke --plan / etc. switched to fallout.
• docs/05-cicd/github-actions.md — generated-output YAML examples updated to the 3-step shape; cache-step actions/cache@v2 bumped to @v4 with the current key files; actions/upload-artifact@v1 bumped to @v5 to match the actual generator output; new note explaining the dotnet-tools.json requirement.
• docs/06-global-tool/00-shell-completion.md — shell completion scripts rebranded (command name + function names: _nuke_zsh_complete → _fallout_zsh_complete, etc.).
• docs/06-global-tool/03-navigation.md — navigation function names rebranded (nuke. → fallout., etc.).

Verification

• Edits don't change link targets, only prose and code examples.
• Docusaurus onBrokenLinks: 'throw' will catch any regression at deploy time.
• PR-C is docs-only, so the noop ubuntu-latest-docs.yml workflow handles the GitHub PR check.

Test plan

• ubuntu-latest (docs-only noop) goes green.
• After merge, docs.fallout.build deploy succeeds (no broken-link regressions thrown by Docusaurus).
• Spot-check a couple of pages on the live site: Build Setup, Build Execution, GitHub Actions, Shell Completion.

Refs

• Closes Docs CLI cleanup: replace stale nuke references with fallout across docs/ #198 (docs CLI cleanup).
• Completes PR-C of Deprecate per-repo bootstrappers; make Fallout.GlobalTool the canonical entry point #203 (bootstrapper-deprecation plan). PR-A (feat(globaltool): add in-tool build runner (PR-A of bootstrapper deprecation) #201) and PR-B (feat(globaltool)!: PR-B — thin shims + local tool manifest + workflow regen (#203) #204) are merged.
• Companion issue Add migration helper docs: alias nuke → fallout CLI per OS/shell #199 (CLI alias bridge guide for migrating users) is a separate enhancement, not in scope here.