SEO: Add canonical URLs

[blyonng]

Q: What is a canonical tag?
A: In order to avoid duplicate content penalties, we need to specify what the original URL is for a piece of content.

Ex:

1. If ngrok.com/docs/foo has the same or similar content to ngrok.com/docs/bar, we need to specify that ngrok.com/docs/foo is the more important page.
2. If ngrok.com/docs/foo is linked to from another website using URL parameters like ngrok.com/docs/foo?asdf=123, then we need to indicate that those are the same page and not different URLs via the canonical tag.

https://developers.google.com/search/docs/crawling-indexing/consolidate-duplicate-urls

Suggestion:

1. Add a field that can be used to specify a canonical URL for a piece of content.
2. If that isn't specified, then auto-generate a canonical tag based on the slug.