docs: add feature matrix

Probably incomplete and inaccurate, we will iterate on it.

Fixes #2344

docs/content/docs/reference/matrix.md

@@ -0,0 +1,43 @@
++++
+title = "Feature Matrix"
+description = "Matrix showing which FTL features are supported by each language"
+date = 2021-05-01T08:20:00+00:00
+updated = 2021-05-01T08:20:00+00:00
+draft = false
+weight = 120
+sort_by = "weight"
+template = "docs/page.html"
+
+[extra]
+toc = true
+top = false
++++
+
+| System        | Feature         | Go  | JVM | Rust |
+| :------------ | :-------------- | :-- | :-- | :--- |
+| **Types**     | Basic Types     | ✔️  | ✔️  | ️ ✔️ |
+|               | Optional Type   | ✔️  | ✔️  |      |
+|               | Unit Type       | ✔️  | ✔️  |      |
+|               | Empty Type      | ✔️  | ✔️  |      |
+|               | Generic Types   | ✔️  | ️   |      |
+|               | Type Aliases    | ✔️  | ️   |      |
+|               | Value Enums     | ✔️  | ️   |      |
+|               | Type Enums      | ✔️  | ️   |      |
+|               | Visibility      | ✔️  | ✔️  |      |
+| **Verbs**     | Verb            | ✔️  | ✔️  | ️✔️  |
+|               | Sink            | ✔️  | ✔️  |      |
+|               | Source          | ✔️  | ✔️  |      |
+|               | Empty           | ✔️  | ✔️  |      |
+|               | Visibility      | ✔️  | ✔️  |      |
+| **Core**      | FSM             | ✔️  | ️   |      |
+|               | Leases          | ✔️  | ✔️  |      |
+|               | Cron            | ✔️  |     |      |
+|               | Config          | ✔️  | ✔️  |      |
+|               | Secrets         | ✔️  |     |      |
+|               | HTTP Ingress    | ✔️  | ✔️  |      |
+| **Resources** | PostgreSQL      | ✔️  | ️   |      |
+|               | MySQL           |     |     |      |
+|               | Kafka           |     |     |      |
+| **PubSub**    | Declaring Topic | ✔️  | ✔️  |      |
+|               | Subscribing     | ✔️  | ✔️  |      |
+|               | Publishing      | ✔️  | ✔️  |      |