chore: switch mkdocs dependency to mkdocs-ng

[shenxianpeng]

Hi, this is a small PR to suggest switching the mkdocs dependency to mkdocs-ng.

The mkdocs package on PyPI is currently unmaintained — see Is MkDocs still maintained? and The Slow Collapse of MkDocs for context.

mkdocs-ng is a community-maintained fork that:

• Keeps the same mkdocs CLI — no changes for end users
• Adds Python 3.13 / 3.14 support
• Includes bug fixes and active development
• One-line drop-in replacement

- mkdocs
+ mkdocs-ng

No obligation — just sharing in case it's helpful. Thanks!