docs(vm): add VM catalog documentation

[coulof]

Problem:

Provisioning standard VMs currently requires manually configuring CPU, memory, disks, networks, and OS settings through the full VM creation form (as discussed in harvester/harvester#11101). With the introduction of the optional VM Catalog add-on, comprehensive documentation is needed.

Solution:

Add docs/vm/vm-catalog.md documenting the new VM Catalog workflow:

• Architectural approach: Built directly on upstream KubeVirt CRDs (VirtualMachineClusterPreference and VirtualMachineClusterInstancetype) rather than custom Harvester catalog CRDs, delegating resolution to KubeVirt's native expand-vm-spec subresource API.
• Top-level sidebar placement: Positioned directly in the main sidebar under root as Catalog when the vm-catalog add-on is enabled under Advanced > Addons.
• Instance types: Series breakdown (u1, o1, cx1, m1, n1, rt1) and size tiers.
• Image preparation: Required labels (instancetype.kubevirt.io/default-preference, instancetype.kubevirt.io/default-instancetype, harvesterhci.io/os-type), recommended Linux distributions, and air-gapped staging with Hauler.
• Creation guide: Step-by-step instructions with cloud-init SSH/password injection and specification preview.
• Windows golden images: Preparation with Sysprep answer files, VirtIO storage driver, and EFI/TPM preferences.
• Known limitations & AddOn management: Enabling/disabling via Advanced > Addons.

Related Issue(s):

harvester/harvester#11101

Related PRs:

• Core / AddOn Packaging: feat: add vm-catalog addon packaging and manifests harvester#11765
• UI Extension: feat: add VM catalog driven by KubeVirt instancetypes and preferences harvester-ui-extension#1171
• UI / E2E Tests: feat: add VM catalog UI and navigation tests harvester-ui-tests#73

Test plan:

1. Verify Docusaurus builds cleanly.
2. Verify sidebar positioning and anchor links.

AI Usage Disclosure

• AI tools were used in preparing this PR.
  • Tools Used: OpenCode (Google Gemini 3.8 Flash)
  • Scope of AI Assistance: Documentation drafting for the VM Catalog guide, Markdown structure, and architecture overview.

Author Acknowledgments

• I have thoroughly reviewed and tested all AI-generated or assisted code.
• I fully understand every change included in this PR and can explain its implementation.
• I confirm this PR adheres to the SUSE AI Governance & Safety Mandate and Harvester Developer Guide.
• Commit messages and PR discussion comments are written directly by me without AI generation.