build(mermaid): enable Docusaurus mermaid diagram rendering

Install and register @docusaurus/theme-mermaid and set
markdown.mermaid so fenced mermaid code blocks render as diagrams
instead of plain code. The Hierarchical Queue guide added in this
branch, along with the existing scheduler and performance guides,
rely on these diagrams.

Pin the theme to 3.10.1 to match the Docusaurus core version so a
single @docusaurus/theme-common instance is used. A mixed version
tree produces duplicate ColorModeProvider contexts and breaks
static site generation.

Signed-off-by: Hajnal Máté <hajnalmt@gmail.com>

Author: hajnalmt

docusaurus.config.js

@@ -47,11 +47,14 @@ const config = {
   onBrokenLinks: "throw",
   markdown: {
     format: "detect",
+    mermaid: true,
     hooks: {
       onBrokenMarkdownLinks: "throw",
     },
   },
 
+  themes: ["@docusaurus/theme-mermaid"],
+
   i18n: {
     defaultLocale: "en",
     locales: ["en", "zh-Hans"],