Add canonical link element to pages

[irisli]

https://en.wikipedia.org/wiki/Canonical_link_element

Some pages have multiple ways of getting to. For example, root pages can be accessed via /index.html and /. The canonical link element resolves this "duplication" of content.

For example, https://www.stellar.org/developers/learn/index.html and https://www.stellar.org/developers/learn/index.html would have the following html tag:

<link rel="canonical" href="https://www.stellar.org/developers/learn/" />

[irisli]

Though perhaps the better solution is to have index.html redirect to / and also make sure we don't link to index.html especially since they are technically the same link.

https://www.mattcutts.com/blog/rel-canonical-html-head/