Add Elixir Quickstart & SDK Documentation

[Cliftonz]

We have quickstarts for all our SDKs and libraries. There's a Elixir SDK without a quickstart.

A quickstart needs to be written for Elixir users to quickly get started with the Elixir SDK.

Good examples that can guide you to write a great Elixir Novu quickstart are the Node.js & Kotlin quickstart guides.

We also need an SDK listing for Rust under Server Side.

• Elixir Quickstart
• Elixir SDK Listing under Server Side