Document/Improve include_mmd! file reading and panic message

[mattsse]

currently, the value of the include_mmd attribute (the path) is just directly:

aquamarine/src/attrs.rs

Line 107 in 6701fdb

let data = std::fs::read_to_string(path).expect("Unable to read mermaid file");

though it's unclear what the current path in this context is when invoked via cargo doc in a workspace.

At the very least the panic should include the current working dir and the path that does not exist.

[mattsse]

closed by #47