docs: custom templates on v2

[breardon2011]

Updates the templates docs for v2, and fixes the SDK export the docs depend on.

Why

The templates page said image builds were not supported on v2. They now are — parity-microvm (#703) implements them — so the page described a gap that no longer exists.

Docs

sandboxes/templates.mdx — leads with the contract being unchanged: same Image builder, same names, same Sandbox.create({ template }). Existing definitions carry over as written.

It then covers only what a customer can actually observe:

• Build ahead of time. A build takes minutes, so it is a separate step; passing image: inline to a create is refused rather than timing out or silently returning a sandbox without the packages.
• Template creates cold-start. The warm pool holds standard sandboxes.
• AL2023 on ARM64. aptInstall works and common Debian names are translated (build-essential → gcc gcc-c++ make); a Debian-only package fails the build and the error names it. A step that downloads an x86-64 binary fails.
• 10 templates per org, identical definitions deduped.

An earlier draft offered a choice between "workspace templates" and "custom images". That was wrong — there is one templates path in the API, and "workspace template" named an internal mechanism customers never see. Removed, including a pre-existing use of the term in the migration guide.

Consistency fixes — pages that would otherwise contradict the above: migrating-from-v1.mdx ("Image builds are unavailable" / "no build pipeline in v2") and reference/typescript-sdk/image.mdx, which described the base as Ubuntu 22.04 with apt-get.

SDK fix (why it is in this PR)

Image and Snapshots were exported as types only, so import { Image } from "@opencomputer/sdk" — exactly what the Image reference documents — failed at runtime with "does not provide an export named 'Image'". There was no supported way to define a template through the SDK at all.

Found by the first end-to-end template run, which could not import them. The docs here show that import, so the fix ships alongside. Also on #703; harmless if that merges first.

Validation

The flow these docs describe was run end to end against the dev cell:

snapshots.create({name, image: Image.base().aptInstall(["tree"])})
waitUntilReady(name)                → ready
Sandbox.create({template: name})    → 8.6s cold launch
exec "command -v tree"              → /usr/bin/tree

tree is absent from the base image, so its presence confirms the customer's package came from their template.

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