Adds a section for Elixir clients

[aseigo]

There are two quality, maintained options for Elixir clients, which are now included on the GraphQL clients listing.

[carolstran]

👋🏼 Thanks for taking the time to open this pull request! I agree that Elixir should eventually be represented on this page. But currently, we're re-evaluating how we handle contributions to this Code page and pausing any changes until after those decisions are made. So I'm going to mark this PR as On hold and if you have any thoughts on this topic, it'd be great if you could voice them in #911

[carolstran]

Hey hey 👋🏼 the Gatsby migration (#938) has been merged so now we can update the content again. Could you please resolve the conflicts on your branch?

[aseigo]

'Morning @carolstran ... I've updated the PR, rebased on the current upstream repo. If you need anything else done to it, please let me know! :)

[carolstran]

Looks great, thank you! Merging now 🚀

[aseigo]

❤️ thanks for the merge!