feat: host the README badge at commit-check.com/badge.svg (#46)

This serves the Commit Check badge from the site, so the snippet a user
copies becomes one line:

```markdown
[![commit-check](https://commit-check.com/badge.svg)](https://commit-check.com)
```

Today the snippet is a 519-character shields.io URL with the mark
base64-encoded into it, and every page view makes a request to a third
party. conventionalbranch.org/badge.svg already works this way.

## What's in it

- **`docs/badge.svg`.** This is a hand-written copy of the shields.io
render of the brand badge from
[`branding/README.md`](https://github.com/commit-check/.github/blob/main/branding/README.md).
It has the same 157×20 size, the same colors and flat style, and the
same `textLength`, which keeps the text identical on machines without
Verdana. The mark is drawn as vectors instead of an embedded image. A
comment in the file explains all of this, including the contrast
trade-off: white on Signal Blue is about 3:1, which is the color the
badge has always had. MkDocs copies the file to the site root.
- **Getting started.** A short *Show that you use it* section now shows
the badge, with Markdown and reStructuredText snippets in tabs. Until
now only the READMEs documented the badge.

## Checks

- I rendered both files with `rsvg-convert` at 6× and compared them with
ImageMagick. At 8% fuzz, 0 pixels differ, and the RMSE is 1.5e-5.
- `mkdocs build --strict` passes (with `SOCIAL_CARDS=false`).
`site/badge.svg` is in the output, and the page links it as
`../badge.svg`.
- `python -m pytest tests/ -q` passes with the released commit-check
installed (10 tests). The pins and the changelog were already at 2.18.1,
so nothing needed bringing into step.

## After this deploys

Once `https://commit-check.com/badge.svg` is live, the long snippet
elsewhere can switch to the short one:
- the *Show that you use it* sections of the commit-check and
commit-check-action READMEs (commit-check/commit-check#587,
commit-check/commit-check-action#293);
- the *README badge* section of `branding/README.md` in `.github`.

The badge rows in those READMEs can stay on shields.io. Only the snippet
users copy needs to change.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Added a “Show that you use it” section with a commit-check badge and
copyable Markdown and reStructuredText snippets.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: shenxianpeng

docs/badge.svg

@@ -180,6 +180,27 @@ a release artifact came from this repository's build pipeline:
 $ gh attestation verify commit_check-*.whl --repo commit-check/commit-check
 ```
 
+## Show that you use it
+
+Put the badge in your README, so contributors know their commits will be
+checked before they open a pull request:
+
+[![commit-check](badge.svg)](https://commit-check.com)
+
+=== "Markdown"
+
+    ```markdown
+    [![commit-check](https://commit-check.com/badge.svg)](https://commit-check.com)
+    ```
+
+=== "reStructuredText"
+
+    ```rst
+    .. image:: https://commit-check.com/badge.svg
+       :target: https://commit-check.com
+       :alt: commit-check
+    ```
+
 ## Where to go next
 
 - **[Where to run it](guides/integrations.md)** — pre-commit hook, GitHub