[docs] Prompt Embedding feature support

[qthequartermasterman]

Purpose

#24278 officially enabled support for Prompt Embeddings in the v1 engine, and achieved feature parity with the implementation in the v0 engine. Since it's now available in both engines, I think it makes sense to add Prompt Embeds to the feature matrix in the docs so that it is clearly documented what features will and wont work with them.

Test Plan

N/A

Test Result

N/A

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request aims to add documentation for Prompt Embedding support in the v1 engine. However, my review found a critical contradiction. Other files in the codebase, including a test file and another documentation file, indicate that this feature is not yet supported in v1. This suggests the documentation update may be based on a false premise and could mislead users. I've added a critical comment to highlight this discrepancy.