Add documentation for debugging online

exercises/shared/.docs/debug.md

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+# Debug
+
+When a test fails, a message is displayed describing what went wrong and for which input.
+You can also use the fact that anything printed to "standard out" will be shown too.
+You can write to standard out using `println!`:
+
+```rust
+println!("Debug message");
+```
+
+Use braces `{}` to print the value of a variable or expressions:
+
+```rust
+let secret_number = 4321;
+println!("My banking password is {}, which is better than {}.", secret_number, 1200 + 34);
+```
+
+Note that not all types can be printed this way, only the ones that can be presented to users unambiguously.
+For example, vectors (lists) cannot be printed like this, because it's not clear how a vector should be displayed to users!
+For these other types, you can tell the `println!` macro to use _debug-formatting_ with: `{:?}`.
+This representation is only intended to be seen by developers.
+An example:
+
+```rust
+let words = vec!["hello", "world"];
+println!("words that are often combined: {:?}", words);
+```
+
+There is even the handy `dbg!` macro, which can be used to debug large expressions inline, without having to pick it apart into separate variable declarations.
+
+Assume we have the following expression:
+
+```rust
+"hello world".split(' ').collect::<Vec<_>>().join("-")
+```
+
+Now we would like to see what the call to `collect` returns.
+We could do the following:
+
+```rust
+let my_temporary_debug_variable = "hello world".split(' ').collect::<Vec<_>>();
+println!("{:?}", my_temporary_debug_variable);
+my_temporary_debug_variable.join("-")
+```
+
+But that's tedious!
+We don't wan't to rewrite our nice expressions just to be able to debug them.
+Please welcome to the stage: `dbg!`
+
+```rust
+dbg!(
+    "hello world".split(' ').collect::<Vec<_>>()
+).join("-")
+```
+
+The whitespace is only for clarity.
+You can wrap any expression in `dbg!()`, which will print that value (with debug-formatting).
+Afterwards you can keep using the value in a larger expression, as if the `dbg!` wasn't even there.
+
+Happy debugging!