docs: 优化虚拟内存文章与字符集目录 — 操作系统文档

- 优化虚拟内存文章表述、结构和面试回答
- 调整字符集侧边栏为明确标题入口
- 仅提交文档与目录改动，未运行站点构建

Author: Snailclimb

docs/.vuepress/sidebar/index.ts

@@ -204,7 +204,10 @@ export default sidebar({
           children: [
             "basis",
             "nosql",
-            "character-set",
+            {
+              text: "字符集详解",
+              link: "character-set",
+            },
             {
               text: "SQL",
               icon: ICONS.SQL,