Surface signature information in hover docs

[varungandhi-src]

Clangd has a fair bit of code around handling hover documentation, see clangd/Hover.cpp (also some earlier discussion in #96). This is because signatures can contain a fair bit of information around attributes etc.

At the moment, scip-clang's hover docs purely contain doc comments, no signatures. We should consider adding signatures to this. If signature_documentation is added to SCIP before we get to this (see sourcegraph/scip#154), we should set that field instead of putting the signature in a Markdown block.