Fix documentation.

- Merge guide.md updates from GitHub.
- Update Opus repository.
- Fix missing link in Opus README.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=122080285

extensions/opus/README.md

@@ -36,7 +36,7 @@ NDK_PATH="<path to Android NDK>"
 
 ```
 cd "${OPUS_EXT_PATH}/jni" && \
-git clone git://git.opus-codec.org/opus.git libopus
+git clone https://git.xiph.org/opus.git libopus
 ```
 
 * Run the script to convert arm assembly to NDK compatible format: