Add the go router v14 migration guide

[ValentinVignal]

Adds the migration guide for go router v14 (flutter/packages#6495)

Fixes flutter/flutter#146805

Presubmit checklist

• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• 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).
• This PR uses semantic line breaks of 80 characters or fewer.

[sfshaza2]

/gcbrun

[sfshaza2]

Before I land this, @ValentinVignal, can you finish the doc? It still has template text and isn't complete. thx

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit fadc9b4):

https://flutter-docs-prod--pr10423-patch-3-m3avp5g1.web.app

[ValentinVignal]

Before I land this, @ValentinVignal, can you finish the doc? It still has template text and isn't complete. thx

Oops, sorry about that ! I updated the doc

[domesticmouse]

/gcbrun

[domesticmouse]

Before I land this, @ValentinVignal, can you finish the doc? It still has template text and isn't complete. thx

Oops, sorry about that ! I updated the doc

I looked at the linked doc under incognito and it looks... very empty. Also, the images don't render under incognito.

[ValentinVignal]

I looked at the linked doc under incognito and it looks... very empty. Also, the images don't render under incognito.

Oh weird, I also don't see the image, I don't know what's happening, I just followed the instructions. Let me investigate.

However, I'm able to see the content, not you?

[domesticmouse]

Sorry @ValentinVignal, I'm used to these having a lot more content. If that's all that is required, sweet.

[ValentinVignal]

That's super weird, on the first loading in an incognito tab, it shows the warning icon instead of the images. But if I load it in a 2nd incognito tab, it displays the images

Video

Screen.Recording.2024-04-17.at.9.48.11.PM.mov

[ValentinVignal]

If I start again from scratch:

1. Go to flutter.dev/go/template
2. File > Make a copy
3. Click Share in the top right
4. Click “Change to anyone with the link” on the bottom left of the dialog that appears.
5. Change Viewer to Commenter in the dropdown near the bottom right corner.
6. Click Done.

I still get the same issue 🤔

[sfshaza2]

lgtm