[STREAM] Add Media Transformations documentation for source origin controls

[bkrebsbach]

Summary

Adds documentation for Media Transformations source origin controls, and updates screenshots for both Media and Image Transformations.

Documentation checklist

• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

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PR Change Summary

Added documentation for Media Transformations source origin controls and updated related screenshots.

• Introduced new documentation for defining source origins in Media Transformations.
• Updated screenshots for Media and Image Transformations.
• Clarified the process for adding allowed origins for source videos.

Added Files

• src/content/docs/stream/transform-videos/sources.mdx

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