Generate Rules.md and md###.md files from metadata, improve Parameters documentation by referencing schema.

doc-build/md040.md

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+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. 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>.