Allow mkdocs markdown to reference protobuf-generated code blocks by name

[rothfels]

E.g. in api_cmds.go the [[.code "<url>" lineStart lineEnd]] should be able to reference a protobuf-generated function by name: [[.code "graph/def.pb.go" "DefKey"]]

[sqs]

FYI: Let's use the Hugo-based docs app for the next revision of these docs instead of further customizing mkdocs. Then we can probably make a shortcode for embedded sourceboxes that refer to a specific def. 👍

[rothfels]

👍