Use gdnative-doc for documentation

[arnaudgolfouse]

This adds a dependency on gdnative-doc to generate the documentation from the Rust code.

The actual content of the documentation (previously in addons/dijkstra-map/DOCUMENTATION.md, now in addons/dijkstra-map/doc/index.md and addons/dijkstra-map/doc/DijkstraMap.md) was slightly changed:

• Methods are no longer organized in sections (previously Methods for constructing and modifying the graph-Methods for recalculating the DijkstraMap-etc...). To counter this, I tried to add more class-level explanation.
• There are now links to Godot's own documentation !
• A LOT of examples were added: not only do I hope this will be beneficial to the docs, they also automatically get translated into Gut tests

Overall I think it looks very good, and I hope this will make for good and maintainable documentation 😁

P.S.: Maybe this will require some explanation so that contributors know they need to modify lib.rs to affect the documentation...