Migrate from mkdocs to properdocs

- Replace mkdocs-with-pdf + weasyprint with properdocs>=1.6.7
- Add pymdown-extensions for richer markdown support
- Remove heavy apt dependencies (weasyprint, fonts-noto-cjk, wget, unzip)
- Switch build command from 'mkdocs build' to 'properdocs build --strict'
- Update template: add offline plugin, fix palette, add content.code.select/copy
- Add md_in_html, pymdownx.tilde, emoji, mermaid, annotation icon support
- Fix broken icon path material//weather-sunny

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

requirements.txt

@@ -1,5 +1,5 @@
 mkdocs-material
 mkdocs-same-dir
 mkdocs-minify-plugin
-mkdocs-with-pdf
-weasyprint
+properdocs>=1.6.7
+pymdown-extensions