Add Mediabunny Codec Registry

docs/codec-registry/vp9.md

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+<script setup>
+import { VPBadge } from 'vitepress/theme'
+</script>
+
+<VPBadge type="info" text="Video codec" />
+
+# VP9 codec registration
+
+## Description
+
+The VP9 video codec, specified in the [VP9 Bitstream & Decoding Process Specification](https://storage.googleapis.com/downloads.webmproject.org/docs/vp9/vp9-bitstream-specification-v0.6-20160331-draft.pdf).
+
+## Codec ID
+
+```ts
+'vp9'
+```
+
+## `EncodedPacket` data
+
+The packet's data must be a frame as described in Section 6 of the [VP9 Bitstream & Decoding Process Specification](https://storage.googleapis.com/downloads.webmproject.org/docs/vp9/vp9-bitstream-specification-v0.6-20160331-draft.pdf).
+
+## `EncodedPacket` type
+
+If the packet's type is `'key'`, then the packet is expected to contain a frame with `frame_type` of `KEY_FRAME`, as defined in Section 7.2 of the [VP9 Bitstream & Decoding Process Specification](https://storage.googleapis.com/downloads.webmproject.org/docs/vp9/vp9-bitstream-specification-v0.6-20160331-draft.pdf).
+
+## `VideoDecoderConfig` codec string
+
+The full codec string begins with the prefix `'vp09.'`, with a variable-length suffix as specified in the [VP Codec ISO Media File Format Binding](https://www.webmproject.org/vp9/mp4/).
+
+## `VideoDecoderConfig` description
+
+`description` is not used for this codec.