build(deps): bump @mdit/plugin-figure from 0.16.0 to 0.21.0 in /docs

Bumps [@mdit/plugin-figure](https://github.com/mdit-plugins/mdit-plugins/tree/HEAD/packages/figure) from 0.16.0 to 0.21.0.
- [Release notes](https://github.com/mdit-plugins/mdit-plugins/releases)
- [Changelog](https://github.com/mdit-plugins/mdit-plugins/blob/main/packages/figure/CHANGELOG.md)
- [Commits](https://github.com/mdit-plugins/mdit-plugins/commits/v0.21.0/packages/figure)

---
updated-dependencies:
- dependency-name: "@mdit/plugin-figure"
  dependency-version: 0.21.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

docs/package.json

@@ -6,7 +6,7 @@
     "preview": "vitepress preview"
   },
   "dependencies": {
-    "@mdit/plugin-figure": "^0.16.0",
+    "@mdit/plugin-figure": "^0.21.0",
     "@mdit/plugin-tasklist": "^0.16.0",
     "aplos": "workspace:*"
   },