docs(route-groups): add an example about opting loading skeletons to a specific route

[samcx]

Why?

Add an additional example that highlights using Route Groups to opt a loading skeleton to a specific route.

• Fixes: Docs: Missing documentation about route groups and loading page behavior #71321

[samcx]

• docs(route-groups): add an example about opting loading skeletons to a specific route #71328 👈
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[gmoniava]

To create a loading skeleton via a loading.js file for a specific route, create a new route group (e.g., /(overview)) and then move your loading.tsx inside that route group

This explanation is not clear. You can already create loading skeleton via loading.js for a specific route - this explanation does not explain what route groups add to that. It should elaborate in which situations is combination of route groups and loading.js files useful.

@samcx @ijjk

[github-actions]

All broken links are now fixed, thank you!

[gmoniava]

See my comment to ztanner.

[samcx]

My gosh Graphite