Add go link for the video_player web Shaka Player design doc

[nateshmbhat]

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

Adds a flutter.dev/go/video-player-web-shaka redirect for the design
document "Shaka Player integration for video_player web".

The document proposes a Shaka Player-backed VideoPlayerPlatform
implementation for Flutter web, which would add HLS and DASH playback plus
audio-track and video-quality selection to video_player on the web. The
go link is required by the Flutter design document process before the
corresponding design doc issue can be filed against flutter/flutter.

The new entry sits in alphabetical order between /go/video-player-drm
and /go/view-metrics-overrides.

Issues fixed by this PR (if any):

Relates to flutter/flutter#66931.

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

None.

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]

Code Review

This pull request adds a new 301 redirect rule in firebase.json mapping the path /go/video-player-web-shaka to a Google Document. There are no review comments, and I have no feedback to provide.

[flutter-website-bot]

Staged preview of the updated docs.flutter.dev site (updated for commit 9b6087c):

https://flutter-docs-prod--docs-pr13862-go-link-video-player-w-5wmvxzkf.web.app

[flutter-website-bot]

Staged preview of the updated flutter.dev site (updated for commit 9b6087c):

https://flutter-dev-230821--www-pr13862-go-link-video-player-w-bmtvalus.web.app