* docs: update message content types link and add content types table
- Update "See All Message Values" link to point to main branch (line 664)
instead of outdated commit 8600ec7 (line 392)
- Add Content Types table documenting all 6 multimodal content types:
text, image_url, input_audio, video_url, file, document
- Link to existing docs for vision, audio, and document understanding
* docs: add type definition links for text and video_url
* docs: fix text type definition link to line 598
* docs: remove provider labels from file/document types
* docs: add examples for all content types per review feedback