feat: add loading skeletons to template/extension listing pages

[ulises-jeremias]

Describe the feature

Show skeleton placeholders while catalog JSON loads.

Use Case

Blank flash on slow networks looks broken.

Proposed Solution

Lightweight skeleton rows/cards matching final layout (no heavy card redesign).

Other Information

Maintainer-seeded Hacktoberfest issue.

Acknowledgements

• I may be able to implement this feature request
• This feature might incur a breaking change

Version used

website @ main

Environment details (OS name and version, etc.)

n/a (docs/site contribution)