chore(ci): 统一发布工作流命名为 release

- 将工作流文件从 pdf.yml 更名为 release.yml，避免名称与
  实际职责不一致
- 同步更新 README 徽章链接，防止状态展示失效并减少使用者
  困惑

Author: jinzhongjia

.github/workflows/pdf.yml .github/workflows/release.yml.github/workflows/pdf.yml renamed to .github/workflows/release.yml

@@ -4,7 +4,7 @@
 [![build](https://github.com/zigcc/zig-course/actions/workflows/build.yml/badge.svg)](https://github.com/zigcc/zig-course/actions/workflows/build.yml)
 [![check](https://github.com/zigcc/zig-course/actions/workflows/check.yml/badge.svg)](https://github.com/zigcc/zig-course/actions/workflows/check.yml)
 [![deploy](https://github.com/zigcc/zig-course/actions/workflows/deploy.yml/badge.svg)](https://github.com/zigcc/zig-course/actions/workflows/deploy.yml)
-[![pdf](https://github.com/zigcc/zig-course/actions/workflows/pdf.yml/badge.svg)](https://github.com/zigcc/zig-course/actions/workflows/pdf.yml)
+[![release](https://github.com/zigcc/zig-course/actions/workflows/release.yml/badge.svg)](https://github.com/zigcc/zig-course/actions/workflows/release.yml)
 
 > Zig is a general-purpose programming language and toolchain for maintaining robust, optimal and reusable software.
 >