Add Mediabunny Codec Registry

docs/codec-registry/ac3.md

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+<script setup>
+import { VPBadge } from 'vitepress/theme'
+</script>
+
+<VPBadge type="info" text="Audio codec" />
+
+# AC-3 codec registration
+
+## Description
+
+The Dolby Digital (AC-3) audio codec, specified in [ETSI TS 102 366](https://www.etsi.org/deliver/etsi_ts/102300_102399/102366/01.04.01_60/ts_102366v010401p.pdf).
+
+## Codec ID
+
+```ts
+'ac3'
+```
+
+## `EncodedPacket` data
+
+The packet's data must be a sync frame (syntactic element `syncframe()`) as defined in Section 4.3 of [ETSI TS 102 366](https://www.etsi.org/deliver/etsi_ts/102300_102399/102366/01.04.01_60/ts_102366v010401p.pdf), beginning with the sync word `0x0B77`.
+
+## `EncodedPacket` type
+
+The packet's type is always `'key'`.
+
+## `AudioDecoderConfig` codec string
+
+```ts
+'ac-3'
+```
+
+## `AudioDecoderConfig` description
+
+`description` is not used for this codec.