Add comprehensive documentation with i18n support (EN/BG)

- Add docs/ folder with English (en/) and Bulgarian (bg/) documentation
- Create index.md with language switching and links
- English docs: installation, quickstart, architecture, baraql, storage,
  schema, lsm, btree, vector, graph, fts, columnar, transactions,
  distributed, protocol, udf, api-binary, api-http, api-websocket
- Bulgarian docs: installation, quickstart, architecture, baraql,
  schema, lsm, btree, vector, graph, fts, transactions, distributed
- Update README license to BSD 3-Clause
- Add LICENSE file with BSD 3-Clause text

docs/en/udf.md

@@ -0,0 +1,56 @@
+# User Defined Functions
+
+Extend BaraQL with custom functions.
+
+## Usage
+
+```nim
+import barabadb/query/udf
+
+var reg = newUDFRegistry()
+
+# Register standard library
+reg.registerStdlib()  # abs, sqrt, pow, lower, upper, len, trim, substr, toString, toInt
+
+# Custom function
+reg.register("greet", @[UDFParam(name: "name", typeName: "str")],
+  "str", proc(args: seq[Value]): Value =
+    return Value(kind: vkString, strVal: "Hello, " & args[0].strVal & "!"))
+```
+
+## Standard Library Functions
+
+| Function | Description | Example |
+|----------|-------------|---------|
+| `abs(n)` | Absolute value | `abs(-5)` → 5 |
+| `sqrt(n)` | Square root | `sqrt(16)` → 4 |
+| `pow(n, e)` | Power | `pow(2, 3)` → 8 |
+| `lower(s)` | Lowercase | `lower('ABC')` → 'abc' |
+| `upper(s)` | Uppercase | `upper('abc')` → 'ABC' |
+| `len(s)` | Length | `len('hello')` → 5 |
+| `trim(s)` | Trim whitespace | `trim(' hello ')` → 'hello' |
+| `substr(s, start, len)` | Substring | `substr('hello', 0, 3)` → 'hel' |
+| `toString(n)` | Convert to string | `toString(123)` → '123' |
+| `toInt(s)` | Convert to integer | `toInt('123')` → 123 |
+
+## Function Registration
+
+```nim
+reg.register(
+  name: "my_function",
+  params: @[
+    UDFParam(name: "arg1", typeName: "str"),
+    UDFParam(name: "arg2", typeName: "int32")
+  ],
+  returnType: "str",
+  body: proc(args: seq[Value]): Value =
+    # Implementation
+    result = Value(kind: vkString, strVal: "")
+)
+```
+
+## Using UDFs in Queries
+
+```sql
+SELECT greet(name) FROM users;
+```