Add release notes for 1.7.0 release (#9165)

Author: timotheeguerin

website/src/content/docs/docs/release-notes/release-2025-12-09.md

@@ -0,0 +1,53 @@
+---
+title: "1.7.0"
+releaseDate: 2025-12-09
+version: "1.7.0"
+---
+
+# 1.7.0
+
+## Features
+
+### @typespec/compiler
+
+- [#9002](https://github.com/microsoft/typespec/pull/9002) Add `commaDelimited` and `newlineDelimited` values to `ArrayEncoding` enum for serializing arrays with comma and newline delimiters
+- [#8942](https://github.com/microsoft/typespec/pull/8942) - Add 'exit' final event for linter rules
+  - Support 'async' in linter definition and async function as callback for 'exit' event.
+- [#9024](https://github.com/microsoft/typespec/pull/9024) [API] Add `node` to `SourceModel` type
+- [#8619](https://github.com/microsoft/typespec/pull/8619) Add support for escaping param like tags(`@param`, `@prop`, etc.) identifier with backtick in doc comments to allow special characters
+
+### @typespec/http
+
+- [#8962](https://github.com/microsoft/typespec/pull/8962) support documentation on union variants for response descriptions
+
+### @typespec/openapi3
+
+- [#9002](https://github.com/microsoft/typespec/pull/9002) Add `commaDelimited` and `newlineDelimited` values to `ArrayEncoding` enum for serializing arrays with comma and newline delimiters
+
+### @typespec/json-schema
+
+- [#9038](https://github.com/microsoft/typespec/pull/9038) Add discriminator support and polymorphic models strategy option
+  - Automatically injects discriminator property into base models with `@discriminator` decorator
+  - Marks discriminator property as required in generated schemas
+  - New `polymorphic-models-strategy` emitter option with three strategies:
+    - `ignore`: Emit as regular object schema (default)
+    - `oneOf`: Emit oneOf schema for closed discriminated unions
+    - `anyOf`: Emit anyOf schema for open discriminated unions
+  - Includes discriminator.mapping in oneOf/anyOf schemas for improved validation
+
+## Bug Fixes
+
+### @typespec/compiler
+
+- [#8917](https://github.com/microsoft/typespec/pull/8917) Add security warning to tsp init CLI documentation for external templates (#8916)
+- [#8997](https://github.com/microsoft/typespec/pull/8997) UnusedUsing Diagnostics are reported as warning instead of hint when there are linters defined in tspconfig.yaml
+
+### @typespec/http
+
+- [#8961](https://github.com/microsoft/typespec/pull/8961) Support nested unions in operation return types
+
+### @typespec/openapi3
+
+- [#9151](https://github.com/microsoft/typespec/pull/9151) Import OpenAPI 3.1/3.2 schemas with contentEncoding: base64 as bytes type with `@encode("base64", string)` decorator
+- [#9076](https://github.com/microsoft/typespec/pull/9076) Respect `@externalDocs` on properties
+- [#8961](https://github.com/microsoft/typespec/pull/8961) Support nested unions in operation return types