docs: add a troubleshooting section and a table of contents

[tas50]

Why

The README documents every option and has good worked examples, but it says nothing about what to do when a run fails. The failures that cost the most time are the ones where the driver's message is accurate and still unhelpful unless you already know GCE:

• Image family ubuntu-2204-lts is not valid - it was not found in project my-gcp-project almost always means a missing image_project, not a wrong family name. Public images live in their own projects.
• Project my-project is not a valid project is usually credentials or a disabled Compute Engine API, because the driver cannot tell a project that does not exist from one it is not authorised for.
• kitchen create hanging at Waiting for server to be ready has three quite different causes — a missing firewall rule, use_private_ip from outside the network, or OS Login ignoring metadata SSH keys — and the fix is different for each.

What this adds

A ## Troubleshooting section in the house style — a bolded lead naming the symptom, then the fix — covering:

• the validation errors the driver raises by name (project, zone/region, image family and name, machine type, disk type, instance name, disk name, no suitable zone)
• the two that come back from GCE rather than the driver: quota, and a disk type a newer machine series rejects
• the hang at Waiting for server to be ready, with the three causes ranked, and a pointer to max_wait_until_ready so it fails in two minutes rather than ten while you work out which one it is
• the two Windows failures: WinRM::WinRMAuthorizationError from connecting as Administrator, and the guest agent password timeout
• finding instances and disks left behind by a Ctrl-C, which the driver's own cleanup cannot handle because Interrupt is not a StandardError
• -l debug as the catch-all

Every quoted message is the driver's actual text, so searching for it lands here.

Also adds a ## Contents list. The page is long enough now that the Configuration tables are hard to find from the top, and this matches kitchen-docker.

No behaviour change; documentation only.

Testing

$ bundle exec cookstyle --chefstyle
18 files inspected, no offenses detected

$ bundle exec rake test
344 examples, 0 failures

$ npx markdownlint-cli2 README.md
Summary: 0 issues in 0 files