Generate documentation redirects for lowercase rule codes (#15564)

## Summary

Resolves #15016.

## Test Plan

Generate the docs with:

```console
uv run --with-requirements docs/requirements-insiders.txt scripts/generate_mkdocs.py
```

and, check whether the mapping was created in `mkdocs.generated.yml` and run the server using:

```console
uvx --with-requirements docs/requirements-insiders.txt -- mkdocs serve -f mkdocs.insiders.yml -o
```

scripts/generate_mkdocs.py

@@ -8,6 +8,7 @@
 import shutil
 import subprocess
 from collections.abc import Sequence
+from itertools import chain
 from pathlib import Path
 from typing import NamedTuple
 
@@ -257,10 +258,18 @@ def main() -> None:
     config["plugins"].append(
         {
             "redirects": {
-                "redirect_maps": {
-                    f"rules/{rule['code']}.md": f"rules/{rule['name']}.md"
-                    for rule in rules
-                },
+                "redirect_maps": dict(
+                    chain.from_iterable(
+                        [
+                            (f"rules/{rule['code']}.md", f"rules/{rule['name']}.md"),
+                            (
+                                f"rules/{rule['code'].lower()}.md",
+                                f"rules/{rule['name']}.md",
+                            ),
+                        ]
+                        for rule in rules
+                    )
+                ),
             },
         },
     )