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/schema.md

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+# Schema System
+
+BaraDB uses a schema-first design with type inheritance and automatic migrations.
+
+## Defining Types
+
+```nim
+import barabadb/schema/schema
+
+var s = newSchema()
+
+let person = newType("Person")
+person.addProperty("name", "str", required = true)
+person.addProperty("age", "int32")
+s.addType("default", person)
+```
+
+## Type Inheritance
+
+```nim
+let employee = newType("Employee")
+employee.setBases(@["Person"])
+employee.addProperty("department", "str")
+s.addType("default", employee)
+
+# Resolve inheritance — Employee gets name, age, department
+let resolved = s.resolveInheritance(employee)
+```
+
+## Schema Operations
+
+### Diff
+
+Compare two schemas:
+
+```nim
+let diff = s.diff(oldSchema, newSchema)
+```
+
+### Migrations
+
+Schema changes are tracked and can generate migration scripts.
+
+## Property Types
+
+| Type | Description |
+|------|-------------|
+| `str` | String |
+| `int32` | 32-bit integer |
+| `int64` | 64-bit integer |
+| `float32` | 32-bit float |
+| `float64` | 64-bit float |
+| `bool` | Boolean |
+| `datetime` | Date/time value |
+| `bytes` | Binary data |