feat: add commenting system

[kaushikgopal]

A common feature for most blogs is the ability to post comments. I haven't personally had too much luck with comments on my personal blog from past experience but I understand that this is something other folks might want. So definitely want to explore some way of easily adding comments.

I'm not entirely sure yet if I want to add this into Henry yet but technologies & standards like Webmentions are definitely making this more enticing.

Links to resources which could inspire a possible solution:

• Comment options: https://aristath.github.io/blog/static-site-comments-using-github-issues-api
• Caching web mentions https://mxb.dev/blog/persistent-build-folders-netlify/
• Comments through Webmentions https://webmention.io/ and Bridgy