docs: add install one-liner to docs-site install pages

[eminwux]

Summary

Verbatim documentation update applied from issue #431.
Mode: verbatim.

Spec from issue

• File: docs/site/getting-started.md — Section: ### Quick Install
• File: docs/site/install/install-linux.md — Section: ## Quick Install
• File: docs/site/install/install-macos.md — Section: ## Quick Install
• Each page now leads with the curl -fsSL https://raw.githubusercontent.com/eminwux/sbsh/main/scripts/install.sh | bash one-liner + env-var note, with the existing platform-specific manual snippet retained in a <details> "Or install manually" block.

Diff vs spec

• Each applied hunk matches the issue's "After" wording character-for-character.
• Only the three named files were touched; manual snippets keep their per-page platform specialization (${OS}-${ARCH}, linux-${ARCH}, darwin-${ARCH}).

Closes #431

[eminwux]

PR #434 Review — docs: add install one-liner to docs-site install pages

LGTM — adds the curl … install.sh | bash one-liner + env-var note to the three install pages with the manual snippets retained in <details>; the documented env vars (SBSH_VERSION, SBSH_INSTALL_PREFIX default /usr/local/bin, SBSH_REPO, SBSH_SKIP_CHECKSUM) and the scripts/install.sh raw URL all match the actual script.

[eminwux]

Superseded by #433 (commit 98e3000, "feat: publish installer at sbsh.io/install.sh with --check/--help and docs one-liner").

#433 already added the install one-liner, env-var override note, and the <details> "Or install manually" collapsible block to all three target files (docs/site/getting-started.md, docs/site/install/install-linux.md, docs/site/install/install-macos.md) — which is what produced the merge conflict here.

It also did so with a better URL: #433 uses the published https://sbsh.io/install.sh and documents --check/--help, whereas this PR uses the raw raw.githubusercontent.com/.../scripts/install.sh link and omits those notes. Merging would be a regression. Issue #431 is already marked completed. Safe to close.