chore: upgrade to MkDocs NG 1.11.0 and Material 9.8.0 (#72)

Bumps the docs toolchain to the current MkDocs NG releases:

- `mkdocs-ng-material[imaging]` 9.7.7 → 9.8.0
- pin `mkdocs-ng==1.11.0` explicitly (the live site was built with
1.9.0, whatever the Material pin resolved to)

Theme, plugins (blog, social) and content are unchanged.

Checked locally:

- `mkdocs build --strict` passes; generator meta is `mkdocs-1.11.0,
mkdocs-material-9.8.0`; 55 pages, social cards still rendered
- `python -m pytest tests/ -q` with the released commit-check: 18
passed, so pins and changelog are in step with PyPI

Branch name doesn't follow the `chore/...` convention because of a
tooling limitation on my side.

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

<!-- coderabbit-ai-generated:v1 -->

## Summary by CodeRabbit

* **Chores**
* Refreshed the documentation site’s build setup with updated tooling
versions. The change is limited to documentation configuration and does
not alter application behavior, user-facing features, or public
interfaces.

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

Author: shenxianpeng

docs/requirements.txt

@@ -4,4 +4,5 @@
 #
 # The `imaging` extra pulls in cairosvg and pillow, which the social plugin
 # needs to render the share cards.
-mkdocs-ng-material[imaging]==9.7.7
+mkdocs-ng==1.11.0
+mkdocs-ng-material[imaging]==9.8.0