format string parser error should point to the docs

[japaric]

For example:

ERROR: Format string error detected.
HELP: `defmt` uses a slightly different syntax than regular formatting in Rust.
  See https://defmt.ferrous-systems.com/macros.html for more details.

[jonathanpallant]

Currently you see:

defmt::println!("Hello, world!, p1 = {:z}", p);

error: unknown display hint: "z"
  --> src/bin/hello.rs:12:21
   |
12 |     defmt::println!("Hello, world!, p1 = {:z}", p);
   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^

error: could not compile `test-pointers` due to previous error

[jonathanpallant]

I have something that now shows:

error: unknown display hint: `"z"`

         = help: `defmt` uses a slightly different syntax than regular formatting in Rust. See https://defmt.ferrous-systems.com/macros.html for more details.

  --> src/bin/hello.rs:12:21
   |
12 |     defmt::println!("Hello, world!, p1 = {:z}", p);
   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^

error: could not compile `test-pointers` due to previous error