docs: add guides for new machines, scripts and CI

[mindsers]

Summary

Adds two task-oriented guides to the documentation site. The tutorial covers a first repository on one machine; these pages cover what users do next.

Set up a new machine (guides/new-machine.md)

• Installing configfile, and giving the machine access to a private repository (SSH key or the host's CLI; no token in the URL).
• init, then modules status and a --dry-run before deploying every module, what happens to the files already in place, and the setup script.
• Copying local files into each project.
• A bootstrap script that sets a machine up without any question (--repo, --all, set -e).
• Keeping the machine in sync: update, deploying new files, removing what the repository no longer deploys.

Use configfile in scripts and CI (guides/scripts-and-ci.md)

• Without a terminal, configfile can't ask questions: a table of each question and the option that replaces it, with the error users get otherwise.
• Exit codes (including git's own, and a script's exit code with scripts run), and output (scripts run keeps stdout for the script; history --json).
• A GitHub Actions workflow that checks a dotfiles repository on every push: it installs configfile, uses a throwaway home folder, and runs modules deploy --all --dry-run, which fails on invalid JSON, missing targets or refused targets. An optional matrix step runs the setup script on Linux and macOS.

The home page links to the guides, and Upgrading from 0.3 now comes last in the Guides section.

Other Informations

• Accuracy: every command, message and exit code shown was run without a terminal in a throwaway home folder: each question's error, deploy --local with and without --force, a dry run on a repository where one module is valid and others aren't (it fails with exit code 1), and an init from a checkout in detached HEAD, as actions/checkout produces for pull requests.
• The example workflow passes actionlint. It sets HOME through $GITHUB_ENV so that every following step uses the throwaway home folder.
• Scope: documentation only.

Testing: pnpm docs:build, the site's type check and pnpm check pass; every internal link and section anchor of the built site resolves.