Provide a link to gh-pages hosted docs (#24)

Swift Package Index is currently building docs in the 6.1
build, which doesn't work for this packge. We can self-host
until that changes.

Author: natecook1000

.spi.yml

@@ -1,4 +1,3 @@
 version: 1
-builder:
-  configs:
-    - documentation_targets: [BinaryParsing]
+external_links:
+  documentation: "https://apple.github.io/swift-binary-parsing/documentation/binaryparsing/"