docs: correct site guidance and complete the first-machine walkthrough

[bketelsen]

The first-machine guide left readers inside the guest before showing host-side commands, and several pages misstated command behavior or used stale versions. This change makes the walkthrough show successful creation, the translated directory, returning to the host, and where to find logs and recovery help.

• Correct required create arguments, VM image revision, root-session behavior, log scope, host-share availability, and the writable-home trust qualification. Include --isolated in CLI help and create usage errors.
• Document arm64 limitations, a passing doctor check, troubleshooting, and uninstall/state retention. Release downloads use the stable releases/latest/download/ names from build: name release archives without the version #51.
• Link the home page and README to one first-machine walkthrough, keep the Podman example in the machines guide, and keep local image build examples in the contributor guide. Add terminology, guide/reference links, navigation cards, and decorative image alt text.

Review decisions: the factual findings and first-run improvements are sound. Keep Homebrew first: the live tap cask already installs v0.7.0, matching the latest release; remove the obsolete future-availability wording instead. The short requirements-table row renders correctly, so its change is only source formatting. The existing desktop-lifetime wording remains correct.

Validation: make site (strict), mise exec -- make ci (including race tests and cross builds), compiled CLI help inspection, rendered HTML checks for table structure/troubleshooting links/decorative alt, and git diff --check. No image or runtime behavior changes; no VM integration run or visual contrast audit.

Rebased onto main (3c9eeb6), 2026-10-05.

• Conflicts: main.go and the CLI reference keep both the PR's --isolated usage and guide links and the --json flags from feat: add --json output to list, images and config #52. The install page keeps build: name release archives without the version #51's version-free download names instead of the X.Y.Z placeholder, plus this PR's arm64 sentence and mkdir -p. The README takes this PR's shorter install line.
• Also updated: the build page's Debian machine image example goes from r4 to r5, because feat(images): report the working directory to VTE terminals #53 raised every machine profile's revision. VM r10 is still current.
• Checks: make ci and the strict make site build pass. nsl help shows --isolated and --json. releases/latest/download/nsl_linux_amd64.tar.gz and checksums.txt resolve against v0.8.0.