docs(site): add English rules page + home intro (Stage 2 of the rules)

site/rules.md is a faithful English mirror of the authoritative Russian
site/ru/rules.md — the same section anchors (so the in-page cross-links
and the RU/EN structure line up), the same LaTeX formulas with English
labels, and the same tables and engine nuances. Rewrite the English home
intro to match the Russian one and link to the rules, and register Rules
in the English sidebar. Completes the bilingual rules.

site/.vitepress/config.ts

@@ -36,7 +36,10 @@ export default defineConfig({
         sidebar: [
           {
             text: "Galaxy",
-            items: [{ text: "Overview", link: "/" }],
+            items: [
+              { text: "Overview", link: "/" },
+              { text: "Rules", link: "/rules" },
+            ],
           },
         ],
       },