Better documentation and examples

[LiamM32]

The documentation is currently really lacking in anything that can teach someone how to use this library to process D code. Some inspiration can be taken from the Phobos documentation to improve this, but more examples would be the first step.

[WebFreak001]

for (larger) examples you can right now look at code from

• https://github.com/dlang-community/dfmt (mostly token processing)
• https://github.com/dlang-community/D-Scanner (lots of simple static AST scanners which will typically be what you write with this library)
• https://github.com/dlang-community/DCD (more advanced indexing and semantic analysis + library "dsymbol" to do it, for symbol resolving)