docs(site): port game rules to site/ru/rules.md (verbatim markdown)

Faithful Markdown rendering of game/rules.txt for the site: headings with
stable anchors, GFM tables and LaTeX formulas — the text itself is
unchanged (typos, the TODO/WTF notes, the broken (---ссылка---) marker and
the lore intro are all preserved as-is). The editorial pass (clarity,
nuances, organic cross-links, intro moved to the home page) follows in a
separate commit so its diff isolates exactly what changed relative to the
original. Registers the page in the RU sidebar.

site/.vitepress/config.ts

@@ -50,7 +50,10 @@ export default defineConfig({
         sidebar: [
           {
             text: "Galaxy",
-            items: [{ text: "Обзор", link: "/ru/" }],
+            items: [
+              { text: "Обзор", link: "/ru/" },
+              { text: "Правила", link: "/ru/rules" },
+            ],
           },
         ],
       },