Transform code blocks from rustdoc into markdown
Rewrite code block start tags, changing rustdoc into equivalent in markdown:
- "", "
norun", "ignore" and "
shouldpanic" are converted to "```rust"
- markdown heading are indentend to be one level lower, so the crate name is at the top level
Code taken almost verbatim from https://github.com/livioribeiro/cargo-readme/blob/a7fd456433832f873cee890a9e67ece9929fc795/src/readme/process.rs.
License: Apache-2.0