[docs] Add rule short code to mkdocs tags (astral-sh#14040)

## Summary

<!-- What's the purpose of the change? What does it do, and why? -->
This PR updates the metadata in the YAML frontmatter of the mkdocs
documentation to include the rule short code as a tag, so it can be
easily searched.
Ref: astral-sh#13684

## Test Plan

<!-- How was it tested? -->
This has been tested locally using the documentation provided
[here](https://docs.astral.sh/ruff/contributing/#mkdocs) for generating
docs.

This generates docs that now have the tags section:
```markdown
---
description: Checks for abstract classes without abstract methods.
tags:
- B024
---

# abstract-base-class-without-abstract-method (B024)
... trimmed
```

I've also verified that this gives the ability to get straight to the
page via search when serving mkdocs locally.

---------

Co-authored-by: Charlie Marsh <[email protected]>

scripts/generate_mkdocs.py

@@ -104,31 +104,68 @@ def clean_file_content(content: str, title: str) -> str:
     return f"# {title}\n\n" + content
 
 
-def add_meta_description(rule_doc: Path) -> str:
-    """Add a meta description to the rule doc."""
-    # Read the rule doc into lines
+def generate_rule_metadata(rule_doc: Path) -> None:
+    """Add frontmatter metadata containing a rule's code and description.
+
+    For example:
+    ```yaml
+    ---
+    description: Checks for abstract classes without abstract methods.
+    tags:
+    - B024
+    ---
+    ```
+    """
+    # Read the rule doc into lines.
     with rule_doc.open("r", encoding="utf-8") as f:
         lines = f.readlines()
 
-    # Get the description from the rule doc lines
+    # Get the description and rule code from the rule doc lines.
+    rule_code = None
+    description = None
     what_it_does_found = False
     for line in lines:
         if line == "\n":
             continue
 
+        # Assume that the only first-level heading is the rule title and code.
+        #
+        # For example: given `# abstract-base-class-without-abstract-method (B024)`,
+        # extract the rule code (`B024`).
+        if line.startswith("# "):
+            rule_code = line.strip().rsplit("(", 1)
+            rule_code = rule_code[1][:-1]
+
         if line.startswith("## What it does"):
             what_it_does_found = True
             continue  # Skip the '## What it does' line
 
-        if what_it_does_found:
+        if what_it_does_found and not description:
             description = line.removesuffix("\n")
+
+        if all([rule_code, description]):
             break
     else:
+        if not rule_code:
+            raise ValueError("Missing title line")
+
         if not what_it_does_found:
             raise ValueError(f"Missing '## What it does' in {rule_doc}")
 
     with rule_doc.open("w", encoding="utf-8") as f:
-        f.writelines("\n".join(["---", f"description: {description}", "---", "", ""]))
+        f.writelines(
+            "\n".join(
+                [
+                    "---",
+                    f"description: {description}",
+                    "tags:",
+                    f"- {rule_code}",
+                    "---",
+                    "",
+                    "",
+                ]
+            )
+        )
         f.writelines(lines)
 
 
@@ -198,7 +235,7 @@ def main() -> None:
         # support.
         mdformat.file(rule_doc, extensions=["mkdocs", "admon", "no-escape-text"])
 
-        add_meta_description(rule_doc)
+        generate_rule_metadata(rule_doc)
 
     with Path("mkdocs.template.yml").open(encoding="utf8") as fp:
         config = yaml.safe_load(fp)