fea: katex and disqus support

[machsix]

• Add disqus support
• Add katex support to render latex formula based on the latest document https://katex.org/docs/autorender.html and another theme https://github.com/carsonip/hugo-theme-minos/blob/master/layouts/partials/footer.html. User can either enable katex globally or enable for a post.
• Add css for blockquote
• Add css for table

[VincentTam]

Disqus has a plethora of drawbacks. You may refer to Pelican-Elegant/elegant#288 for links to relevant articles.

I suggest other alternatives like

1. Staticman
2. Netlify comments
3. JAM stack

ℹ️ I've built @staticmanlab, a public Staticman instance.