layout: add alt attribute on logo `img`
layout: add alt attribute on logo `img`

An img element must have an alt attribute, except under certain conditions.

JeremyHe-cn/hexo-theme-apollo,AI-Frontiers/hexo-theme-apollo,pinggod/hexo-theme-apollo,plck/hexo-theme-apollo,geniustanley/hexo-theme-achilles,plck/hexo-theme-apollo,pinggod/hexo-theme-apollo,Tuzkiss/hexo-theme-apollo,Tuzkiss/hexo-theme-apollo,JeremyHe-cn/hexo-theme-apollo,MikeCoder/hexo-theme-gandalfr,MikeCoder/hexo-theme-gandalfr,geniustanley/hexo-theme-achilles