Signature recognition & Adapter robustness

- Keyword-defined section signature recognition is now in a separate file and includes other keyword-defined sections. 
- Section parser adapters now check markdown node type and throw error on mismatch

Nope this is a runtime check for an error in composing parsers consider following

OK: 

```
typedef SectionParser<Parameter, ListSectionAdapter> ParameterParser;
```

Not OK:

```
typedef SectionParser<Parameter, HeaderSectionAdapter> ParameterParser;
```