Add back(?) introductory video to main docs page

[autorejecttop]

Description of what this PR is changing or adding, and why:

1. Documentation landing page says "... This video walks you through the fundamentals of Flutter and shows you how to get started." yet there is no video about it at all.
2. Looked into the commits, found out that it used to exist in feat: Add introductory video to main docs page #12240 here and was deleted/moved(?) on Remove iOS & Android video from index #12243 here.
3. I added it back to the landing page by copying the id and title to the current video embed format
4. Was it intentional to remove the video? If yes, why not remove the text too?

Issues fixed by this PR (if any):

• None, have checked

PRs or commits this PR depends on (if any):

• feat: Add introductory video to main docs page #12240
• Remove iOS & Android video from index #12243

Presubmit checklist

• If you are unwilling, or unable, to sign the CLA, even for a tiny, one-word PR, please file an issue instead of a PR.
• If this PR is not meant to land until a future stable release, mark it as draft with an explanation.
• This PR follows the Google Developer Documentation Style Guidelines—for example, it doesn't use i.e. or e.g., and it avoids I and we (first-person pronouns).
• This PR uses semantic line breaks
  of 80 characters or fewer.

[gemini-code-assist]

Summary of Changes

Hello @autorejecttop, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses an inconsistency on the main Flutter documentation landing page by reintroducing an introductory video. The page's text previously described a video that was no longer present, and this change ensures that the content aligns with the description, thereby improving the user experience for those seeking a visual guide to getting started with Flutter.

Highlights

• Documentation Inconsistency: The main documentation landing page referenced an introductory video that was not present, creating an inconsistency between the text and available content.
• Video Restoration: This pull request re-adds the introductory video to the main documentation page, resolving the aforementioned inconsistency.
• Historical Context: The author investigated previous commits (feat: Add introductory video to main docs page #12240 and Remove iOS & Android video from index #12243) to understand the history of the video's removal and subsequent absence.
• Author's Query: The author raises a question regarding the intentionality of the video's removal and why the descriptive text was not also removed if the video's absence was deliberate.

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[gemini-code-assist]

Code Review

This PR correctly identifies and fixes an issue where an introductory video was missing despite being mentioned in the text. The change is simple and effective. I have one minor suggestion to improve adherence to the project's style guide regarding line length.

[gemini-code-assist]

This line is 138 characters long, which significantly exceeds the 80-character limit for line length that is part of the repository's style guide (as mentioned in the PR checklist). To improve readability and adhere to the 'semantic line breaks' principle, please format the component to span multiple lines.

Suggested change

	<YouTubeEmbed id="W4JWeQolJsU" title="Build and ship amazing multiplatform iOS and Android apps with one codebase" fullWidth></YouTubeEmbed>
	<YouTubeEmbed
	id="W4JWeQolJsU"
	title="Build and ship amazing multiplatform iOS and Android apps with one codebase"
	fullWidth>
	</YouTubeEmbed>

References

1. The pull request description states adherence to a style guide that uses 'semantic line breaks of 80 characters or fewer'. The added line violates this rule. ^(link)