docs(installation): add manual gateway instructions

[drew]

Summary

Add concise manual gateway and CLI binary setup notes for users who do not want package-manager installation, especially immutable or image-based Linux systems. Move gateway runtime configuration details to a dedicated Gateway Configuration reference page.

Related Issue

Addresses review discussion #1285 (comment)

Changes

• Add manual gateway binary setup with inline target detection and latest-release downloads.
• Add a VM driver note for installing openshell-driver-vm under $HOME/.local/libexec/openshell.
• Add manual CLI binary setup with inline target detection and latest-release downloads.
• Add Gateway Configuration reference page with gateway environment variables and their purpose.
• Link installation and gateway management docs to the new reference.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Additional validation:

• git diff --check
• mise run docs
• mise run markdown:lint

mise run pre-commit did not pass because cargo fmt --all -- --check reported an unrelated formatting diff in crates/openshell-driver-kubernetes/src/driver.rs.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable; not needed for this docs-only install update)

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