docs: replace stale WSL2 references with Hyper-V Linux VM for the serve path

[luthermonson]

Why

The Linux-job dispatch worker on Windows moved from a WSL2 distro to a Hyper-V Linux VM (HCS via vmcompute.dll) at some point and the docs still described the WSL2 architecture. Spotted on https://ephemerd.ephpm.dev/guides/how-it-works/ — but the same stale framing was scattered across the architecture docs, CLI reference, and getting-started pages.

Motivation for the migration, captured in code:

// Unlike the previous WSL2 approach, this works from any Windows security context
// including LocalSystem (services), which WSL2 does not support.

— pkg/vm/linuxvm_windows.go

That sentence is now in the docs too.

What's in vs. out of scope

Updated (the `serve` daemon path on Windows):

• `docs/guides/how-it-works.md` — Windows section rewritten end-to-end (prose + Mermaid + Dual-Purpose Hosts table); plus same-page "One Image" and "Resource planning" tweaks.
• `docs/architecture/windows-wsl-dispatch.md` — page title becomes Windows Hyper-V Dispatch, new Why a Hyper-V VM (not WSL2) section explains the LocalSystem motivation, and the lifecycle prose was rewritten (kernel + initrd + VHDX boot via HCS). Filename intentionally unchanged so existing inbound links and `{{< relref >}}` cross-references keep working; added a Hugo `aliases` entry for the old URL.
• `docs/architecture/overview.md`, `embedded-containerd.md`, `macos-vms.md`, `multi-forge-providers.md`, `forgejo-gitea.md`, `pre-baked-rootfs.md`, `cri-passthrough.md`
• `docs/getting-started/configuration.md`
• `docs/cli/serve.md`
• `docs/guides/providers.md`, `docs/guides/runner-images.md`
• `docs/contributing/dev-setup.md`, `project-layout.md`

Intentionally NOT updated:

• `docs/cli/run.md`, `doctor.md`, `uninstall.md` — these still mention WSL because `ephemerd run` (one-shot CLI invocation) is implemented by `pkg/vm/wslrun_windows.go` and genuinely uses a temporary WSL distro. Same for `doctor`'s WSL availability check and `uninstall`'s cleanup of `ephemerd-*` distros — both still relevant.
• `docs/_archived/...` — explicitly archived, left as historical record.
• `docs/arch/quickboot-image.md` — this is a forward-looking decision doc that already discusses Hyper-V as the target. Could use a status update but that's a separate edit.

Test plan

• Hugo render check on touched pages (no broken `{{< relref >}}` links).
• Old URL `/architecture/windows-wsl-dispatch/` still resolves via the new `aliases` frontmatter.
• No code changes; `mage ci` not required.

Followups if you spot more drift: another wsl grep across docs/ surfaces some `run`-mode + cleanup mentions that I left in place because they're still correct — happy to revisit if the `run` command itself migrates to Hyper-V too.