docs(sys-ext): add comprehensive end-to-end sysext tutorial

[v0idheaven]

Summary

This PR adds a comprehensive, hands-on six-part sysext tutorial to the docs, addressing flatcar/Flatcar#2295.

The existing sys-ext/_index.md is a solid reference page but does not serve as a learning path. New users especially those coming from traditional Linux distros consistently get confused about three things:

• The difference between built-in, official, and community extensions (#1476)
• How to build and test a custom extension end-to-end
• How to deploy it in production and keep it updated

This tutorial fills that gap with working, copy-paste examples at every step.

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New pages

File	Content
sys-ext/tutorial/_index.md	Tutorial landing page with part index
sys-ext/tutorial/understanding-sysext.md	What sysext is, the three extension types, how overlays work
sys-ext/tutorial/using-official-extensions.md	Enabling release extensions, opting out of built-ins, bakery extensions, verifying status
sys-ext/tutorial/building-custom-extension.md	Step-by-step: directory structure, static binary, metadata file, squashfs image, systemd units
sys-ext/tutorial/testing-locally.md	QEMU testing via bakery.sh boot, manual setup, debug table for merge failures
sys-ext/tutorial/production-deployment.md	Ignition provisioning, checksum verification, systemd-sysupdate auto-updates, baking into images
sys-ext/tutorial/contributing-to-bakery.md	Writing a create.sh recipe, smoke tests, PR conventions

Also adds a "New to sysext?" callout at the top of sys-ext/_index.md pointing users to the tutorial.

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What each part covers

Part 1 - Understanding sysext
Explains the read-only overlay model, why Flatcar uses sysext, and gives a clear side-by-side comparison of the three extension types with concrete examples of when to use each.

Part 2 - Using official extensions
Covers enabled-sysext.conf, Butane config examples, opting out of built-in docker/containerd, provisioning bakery extensions, systemd-sysext status, and runtime reload.

Part 3 - Building a custom extension
Full walkthrough packaging jq (a static binary) as a sysext: directory layout, downloading the binary, writing the extension-release metadata, building the squashfs image with mksquashfs, adding systemd units with Upholds= drop-ins.

Part 4 - Testing locally
One-command testing via bakery.sh boot, manual QEMU setup with an HTTP server and Ignition config, and a debugging reference table covering the most common merge errors and how to fix them.

Part 5 - Production deployment
Ignition provisioning with SHA-512 checksum verification, deploying multiple extensions, systemd-sysupdate configuration for automatic updates, and baking extensions into custom Flatcar images.

Part 6 - Contributing to sysext-bakery
Implementing create.sh, adding smoke tests, bakery conventions (no /usr/sbin, static binaries, Upholds= drop-ins), opening a PR, and what happens after merge.

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Closes flatcar/Flatcar#2295

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