Add section about "Cancelability" under "Concepts"

[PaulRBerg]

It would be helpful to add a section about Cancelability in the Concepts section.

We keep getting questions about this:

[PaulRBerg]

This would also clarify situations like this:

[maxdesalle]

https://github.com/sablier-labs/docs/commit/1fe7f88384c5454f53380e9172da67e8577ce6bf