build: standardize build and installation process in documentation and Makefile (#21)

- Add install, uninstall, and clean targets to the Makefile
- Change README instructions from `make build` to `make install`
- Update README.zh-cn instructions from `make build` to `make install`
- Update README.zh-tw instructions from `make build` to `make install`

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/21
Co-authored-by: Bo-Yi Wu <appleboy.tw@gmail.com>
Co-committed-by: Bo-Yi Wu <appleboy.tw@gmail.com>

README.zh-cn.md

@@ -77,7 +77,7 @@ git clone https://gitea.com/gitea/gitea-mcp.git
 然后运行：
 
 ```bash
-make build
+make install
 ```
 
 ### 📁 添加到 PATH