docs: add a troubleshooting section and a table of contents to the README

[tas50]

The README already documents every configuration option with a type and default, and has working kitchen.yml examples for each feature. What it did not have was any help for the moment things go wrong — and Hyper-V has a small, very predictable set of failure modes that catch nearly everyone the first time.

What is new

A Troubleshooting section covering, in the order a new user hits them:

• Start here — the three commands that answer most questions before you read a single error message: kitchen doctor, kitchen diagnose --all, and kitchen create -l debug. Plus dry_run: true, which echoes the generated PowerShell instead of running it.
• The term 'Get-VM' is not recognized / Access denied — the Hyper-V module is missing, or the shell is not elevated. Both are prerequisites and both are easy to miss.
• Failed to find a default VM Switch. — how to list the switches the host actually has, and that vm_switch matches on name.
• Missing parent_vhd_folder / Missing parent_vhd_name — including two things the option table does not say: parent_vhd_name is a file name, not a path, and when hyperv_server is set these paths refer to the remote host, so the local pre-flight check is skipped.
• A create that hangs waiting for an IP address — the three real causes, ordered by how often they are the answer: a private or internal switch with no DHCP behind it, a generation 2 Linux guest that Secure Boot refused to boot, and missing guest integration services.
• A generation 2 VM that will not boot — Secure Boot's default template only trusts Microsoft bootloaders.
• Additional disk file already exists — what leaves one behind and where to find it.
• kitchen destroy cannot find a VM that is still running — Test Kitchen tracks VMs by id, so a lost .kitchen/ orphans them. Includes the PowerShell to clean up by hand.
• WinRM failures against a remote hyperv_server — Enable-PSRemoting, Test-WSMan, and how hyperv_ssl and hyperv_insecure interact.
• copy_vm_files silently doing nothing — it needs enable_guest_services: true.

Also adds a Contents list under the intro, the way kitchen-docker's README has one. At around 400 lines this file is past the point where scrolling is a reasonable way to find the networking options.

Verification

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

$ bundle exec rake test
120 examples, 0 failures

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

Every in-page anchor in the new Contents list and cross-reference resolves to a real heading.