Add schema directive to template manifests

[itowlson]

As of Spin 3.3 we include a spin.toml JSON schema as part of each release. This PR proposes linking that schema from template manifests: this provides (a measure of) validation and code completion out of the box (well, if the user has Even Better TOML or its equivalent installed).

In this PR, I link to the manifest via a versioned release URL. We get this for free, but it does mean we need to update the templates whenever we update the schema doc (or on every release, if we want to avoid folks going "I'm on Spin 3.5, why did you give me this OLD schema"), and that users need to rev the link to get schema updates. If we wanted to invest a little more in the plumbing, we could upload schemas to a version-neutral location so users could automatically inherit improvements (or could still lock to a version via a GH URL). Thoughts?

[lann]

upload schemas to a version-neutral location so users could automatically inherit improvements (or could still lock to a version via a GH URL). Thoughts?

I'm in favor of this. Three options come to mind:

• Github Page from this repo, e.g. spinframework.github.io/spinframework/schemas/...; probably the easiest to get started with since it can be built in this repo
• New subdomain e.g. schemas.spinframework.dev/... which could be hosted by Github Pages pretty easily; would probably make the most sense as a new e.g. spin-schemas repo; technically could be in this repo but that would be a little weird since you can only host one site per repo and schemas would "use up" that slot (though this could be migrated later)
• Existing docs site, e.g. spinframework.dev/schemas/...; would require some integration with https://github.com/spinframework/spin-docs

[lann]

After editing the comment above several times I have developed An Opinion:

Let's create a subdomain like schemas.spinframework.dev pointing at Github Pages built from a branch in this repo. If we ever really want to use this repo's Page for something else we can migrate the schemas somewhere else such as Github Pages in another repo by redirecting the subdomain.

[lann]

^ I'm going to get started on this just because there is some lead time on the DNS update; we can always bail out if we don't like this approach.