Remove empty border space from LDK architecture

The empty border space makes the diagram text smaller than it otherwise
would be. Remove it to make the diagram more readable. Also, move the
file under docs/assets for consistency.

Author: jkczyz

docs/assets/ldk-architecture.svg

@@ -25,7 +25,7 @@ Throughout these docs, if any piece of code is used or mentioned, the most in-de
 found by searching [the Rust docs](https://docs.rs/lightning/0.0.12/lightning/index.html).
 
 ## Architecture
-<img alt="LDK Architecture" src={useBaseUrl('img/ldk_arch.svg')} />
+![LDK Architecture](assets/ldk-architecture.svg)
 
 The big blue box in the center contains all of LDK's core objects.