Add mrdocs configuration

Author: steve-downey

docs/mrdocs.yml

@@ -0,0 +1,18 @@
+---
+source-root: ..
+input:
+  - ../include
+exclude:
+  - ../include/beman/optional/detail
+exclude-patterns:
+  - ../include/beman/optional/detail/**
+includes:
+  - ../include
+file-patterns:
+  - '*.hpp'
+include-symbols:
+  - 'beman::optional::**'
+implementation-defined:
+  - 'beman::optional::detail'
+multipage: true
+generator: adoc