Update SemanticDB guide
Update SemanticDB guide

- New Install section
- More usage examples
- Expand on several notes
- Minor language polish

MasseGuillaume/scalameta,scalameta/scalameta,olafurpg/scalameta,MasseGuillaume/scalameta,xeno-by/scalameta,xeno-by/scalameta,scalameta/scalameta,scalameta/scalameta,scalameta/scalameta,olafurpg/scalameta,scalameta/scalameta,xeno-by/scalameta,MasseGuillaume/scalameta,olafurpg/scalameta