Add SymbolInformation.signature_documentation

Towards #154

Previously, SCIP clients had to parse markdown from the
`SymbolInformation.documentation` field to display only the signature
information of a symbol (for example, method parameters and return
type).

With the new `SymbolInformation.signature_documentation` field, indexers
can now emit more structured information about the signature of a
class/method including optional hyperlinks to the referenced symbols.