Add license to the footer

[longlin10086]

📝 描述

✅ 检查清单

• 是否在合适的文件夹上传？

• 是否获得了必要的许可？

• 文件命名是否符合规范？

• 是否添加了 README.md 文件以帮助其他人理解你的文件？（可选）

🤔 附加信息

In issue #20 @kowyo had raised this proposal, so I just added the license to the page footer.
Here it looks like:

[kowyo]

Thanks for the PR. My origin idea would be adding the license in the end of the article, instead of the bottom of the whole website, which is also a convention of many blog themes. So I might work on it later, anyway, your contribution is still appreciated.

[kowyo]

Will close the PR if 4019e10 looks good to you. Otherwise we can still have some work on it.

[kowyo]

I have referred to the practices of https://creativecommons.org and https://vuejs.org, and believe that placing the copyright notice at the bottom of the page is also a very good practice. Thanks for the PR, @longlin10086.