book: Run code blocks if possible and unify style

[Urhengulas]

Few cleanups for the defmt book

• unify "```rust" and "``` rust` to the latter
• remove ignore attributes from some code blocks, which can be compiled
• add cortex-m to library search path; therefore we don't need to mock it anymore

[Urhengulas]

bors r+

[bors]

Build succeeded:

• ci