docs: clarify video material sources

[HaningZS]

Summary

• explain how Pexels, Pixabay, Coverr, and local files supply existing visual material
• document both paid AI-generation paths: WaveSpeed AI and Shengsuan Cloud / LoomLoom
• add equivalent source explanations to the Chinese, English, and Japanese READMEs
• preserve the current feature-list wording and sponsor sections

Why

The introduction can lead readers to expect that every workflow synthesizes new video frames. The new overview makes the selected video source and shared post-production pipeline explicit, sets license expectations for stock and local material, and covers both AI video integrations currently available on main. It also clarifies the source question raised in #1181 without claiming support for arbitrary creator or channel URLs.

Testing

• rebased onto current upstream/main (05c0baf)
• docs-only boundary check: 33 insertions, 0 deletions across README.md, README-en.md, and README-ja.md
• git diff --check upstream/main...HEAD
• uv run --no-sync python -m compileall -q app cli.py main.py webui test
• uv run --no-sync ruff check app cli.py main.py webui test
• uv run --no-sync python -X utf8 -m coverage run -m pytest -q test: 712 passed, 11 skipped, 6084 subtests passed

Closes #1183

[harry0703]

Thank you for the clear explanation of how stock, local, and AI-generated materials flow through the shared production pipeline. The documentation direction is useful, and I verified that the proposed diff does not alter any sponsor content. The branch is now out of date and the patch no longer applies cleanly to the current README files. Since this PR was opened, main has added WaveSpeed AI as another text-to-video material source and continues to maintain a Japanese README, so merging the current text would make the new overview incomplete across both supported sources and languages. Could you please rebase onto the latest main, cover both WaveSpeed AI and Shengsuan Cloud / LoomLoom in the overview, preserve the current feature-list wording and sponsor sections, and add the equivalent explanation to README-ja.md? Please keep the change documentation-only and focused. Thank you again; we will be happy to review the updated version.

[HaningZS]

Updated as requested—thank you for the detailed review.

• Rebased the branch onto the latest main.
• Added both WaveSpeed AI and Shengsuan Cloud / LoomLoom to the material-source overview.
• Added the equivalent overview to README-ja.md alongside the Chinese and English versions.
• Preserved the current feature-list wording and sponsor sections; the PR now contains only 33 added lines and no deletions across the three README files.
• Re-ran compileall, Ruff, and the full pytest/coverage command locally: 712 passed, 11 skipped, and 6084 subtests passed.

The PR is now mergeable, and the refreshed CI jobs are in progress.