docs: add CLI installation and quick start

[abn]

Summary
Adds installation and first-run instructions for the AetherPak CLI to the README and the project landing page. Docs only, no code changes.

What is new

• README Installation: prerequisites plus three install routes, being the prebuilt release archive for amd64/arm64, a source build with go install ., and the ghcr.io/aetherpak/cli:latest / :latest-builder container images, followed by a verify step using aetherpak --version and aetherpak status.
• README Quick Start: a minimal aetherpak.yaml and a first run through config show, status, publish, release, and add, linking to the existing Command Reference.
• Landing page Installation: the same three routes in a tabbed card that matches the site's existing visual language.
• Correction: the docs no longer claim a gpg binary is required. Signing runs in-process from armored key material.

Impact
No behavior change. New users can go from the project overview to a working first command without reading the source or the CI workflows.

[abn]

I have added install and quick-start docs in two places. The README now has an Installation section (prerequisites, the prebuilt release archive, a source build with go install ., and the container images) and a Quick Start that takes a minimal aetherpak.yaml through the first commands. The landing page gets the same three routes in its existing tabbed card style.

While writing the install notes I corrected a claim that was already on the landing page: the CLI does not need a gpg binary, since signing runs in-process from key material. I fixed the same wording in the new sections.

I verified the release asset URLs, the clone plus go install . flow, both container tags, and every command shown in the examples against the current source and workflows. Ready for review.