Non-invasive mdoc annotations

[saksmt]

Currently only option to provide metadata/annotations to mdoc is to write them in code fences where language is specified, downside is that this breaks syntax highlighting everywhere except github (currently, aaand it fails in nesting - this issue is perfect example).

Instead of waiting for support in IDEA, gitlab and other tools it'd be better to just add another configurations option through comments in either markdown (html/xml or pseudo-link) itself or inside code snippet.

Use cases it enables:

• using mdoc for only validation without any micro-macro-doc-sites whatsoever, just plain markdown in repo
• from what I saw from sources it will require refactoring of parsing which can lead to easier support of formats other than markdown (as requested in Using the library with asciidoc files #491, Define an API for mdoc #536)

---

Current behavior:

```scala mdoc:someMeta
val codeWithBrokenHighlighting = ???
```

Using html/xml comments:

<!-- mdoc someMeta -->
```scala
val codeWithWorkingHighlighting = ???
```

Using preudo-links:

[//]: # (mdoc:someMeta)
```scala
val codeWithWorkingHighlighting = ???
```

Using comments inside code snippet:

```scala
// mdoc someMeta
val codeWithWorkingHighlighting = ???
```