RDF-like Models in Markdown

[vorburger]

After #607, allow embedding that within Markdown:

```yaml !!RDF
BASE: https://enola.dev/markdown
load: https://enola.dev/contexts.yaml
contexts:
  - meta: https://enola.dev/meta
type: meta:Ontology
---
# Markdown

Model of Markdown files.

```yaml !!RDF
emoji: 📃 
---
## Document {#doc}

...

## Links {#link} {:emoji 🔗}

A link to another [#doc] of [enola:] with {properties:}

<!-- (Un)ordered list soon after property name means RDF blank node list? Or just write this in YAML, and let it be replaced by generated? -->

* Target {#target}: 
* Label

The fact that YAML front matter appears not just at the start but also inline can easily be handled by a converter. Note how we're using code blocks marked with a special language. This will get filtered out and replaced by the pre-processor.

Note how links can use prefixes, and just IRI without label.

Note how it's possible to specify properties inline!