docs: add code action videos to IDE features docs (Part of #2040)

[nikmos3]

Summary

This PR updates the IDE-features.mdx documentation by adding video demonstrations and descriptions for two supported Quick Fix code actions:

1. "Add missing import"
2. "Remove redundant cast"

The "Quick Fix" section was refactored slightly to drop the markdown table in favor of the standard **Heading** + description + <video> structure used by other features in the document (such as those in the Refactor section). The two screen recordings (.mp4 format) have been added to the website/static/videos/ directory and are embedded with the standard width="720" attributes to seamlessly align with the layout of existing feature videos.

Part of #2040

• I am a human, and I fully understand the code I am submitting.

Test Plan

• Recorded the Code Action demonstrations using the Pyrefly VS Code extension.
• Processed the videos with ffmpeg (cropping the raw footage and scaling it to 1440px resolution) to optimize playback, and correctly referenced them using root-relative paths (/videos/...) in the MDX file.
• Ran the Docusaurus development server locally (yarn start) and manually verified the UI scaled the videos properly and that they matched the exact style of the other documentation features.

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D124484172. (Because this pull request was imported automatically, there will not be any future comments.)

[asukaminato0721]

I think these are only part of the videos. You can use "part of #xxx" since fix will close the issue

[nikmos3]

I think these are only part of the videos. You can use "part of #xxx" since fix will close the issue

Done