Include heading level overried for deployment applicability

[Mpdreamz]

Implements #94

This introduces a new {applies} directive which closely mirrors the applies: configuration at a page level introduced in #103

## This section has its own applies annotations

```{applies}
:stack: unavailable
:serverless: tech-preview
```

An error will be generated if this directive does not follow a heading for consistency purposes.

cc @bmorelli25 @KOTungseth @lcawl @colleenmcginnis