restore programmability docs

docs/programmability.md

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+# [Programmability](https://redis.io/docs/manual/programmability/)
+
+Redis provides a programming interface allowing code execution on the redis server.
+
+## [Functions](https://redis.io/docs/manual/programmability/functions-intro/)
+
+The following example retrieves a key in redis, returning the value of the key, incremented by an integer. For example, if your key _foo_ has the value _17_ and we run `add('foo', 25)`, it returns the answer to Life, the Universe and Everything.
+
+```lua
+#!lua name=library
+
+redis.register_function {
+  function_name = 'add',
+  callback = function(keys, args) return redis.call('GET', keys[1]) + args[1] end,
+  flags = { 'no-writes' }
+}
+```
+
+Here is the same example, but in a format that can be pasted into the `redis-cli`.
+
+```
+FUNCTION LOAD "#!lua name=library\nredis.register_function{function_name=\"add\", callback=function(keys, args) return redis.call('GET', keys[1])+args[1] end, flags={\"no-writes\"}}"
+```
+
+Load the prior redis function on the _redis server_ before running the example below.
+
+```typescript
+import { createClient } from 'redis';
+
+const client = createClient({
+  functions: {
+    library: {
+      add: {
+        NUMBER_OF_KEYS: 1,
+        FIRST_KEY_INDEX: 1,
+        transformArguments(key: string, toAdd: number): Array<string> {
+          return [key, toAdd.toString()];
+        },
+        transformReply: undefined as unknown as () => NumberReply
+      }
+    }
+  }
+});
+
+await client.connect();
+
+await client.set('key', '1');
+await client.library.add('key', 2); // 3
+```
+
+## [Lua Scripts](https://redis.io/docs/manual/programmability/eval-intro/)
+
+The following is an end-to-end example of the prior concept.
+
+```typescript
+import { createClient, defineScript, NumberReply } from 'redis';
+
+const client = createClient({
+  scripts: {
+    add: defineScript({
+      SCRIPT: 'return redis.call("GET", KEYS[1]) + ARGV[1];',
+      NUMBER_OF_KEYS: 1,
+      FIRST_KEY_INDEX: 1,
+      transformArguments(key: string, toAdd: number): Array<string> {
+        return [key, toAdd.toString()];
+      },
+      transformReply: undefined as unknown as () => NumberReply
+    })
+  }
+});
+
+await client.connect();
+
+await client.set('key', '1');
+await client.add('key', 2); // 3
+```