markdownlint/doc-build/md040.md

This rule is triggered when fenced code blocks are used, but a language isn't
specified:

````markdown
```
#!/bin/bash
echo Hello world
```
````

To fix this, add a language specifier to the code block:

````markdown
```bash
#!/bin/bash
echo Hello world
```
````

To display a code block without syntax highlighting, use:

````markdown
```text
Plain text in a code block
```
````

You can configure the `allowed_languages` parameter to specify a list of
languages code blocks could use. Languages are case sensitive. The default value
is `[]` which means any language specifier is valid.

Rationale: Specifying a language improves content rendering by using the
correct syntax highlighting for code. More information:
<https://cirosantilli.com/markdown-style-guide#option-code-fenced>.