[docs] add <svelte:element> example

[oekazuma]

Add example of <svelte:element> added at #6898

Before submitting the PR, please make sure you do the following

• Prefix your PR title with [feat], [fix], [chore], or [docs].
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with npm test and lint the project with npm run lint

[lukaszpolowczyk]

@oekazuma This thumbnail link does not work:
https://svelte.dev/examples/thumbnails/svelte-element.jpg

This is what it looks like in https://svelte.dev/examples/svelte-element:

[oekazuma]

@lukaszpolowczyk
Thanks for the report.

@tanhauhau @dummdidumm
I do not know how to add thumbnails.
Can you please tell me how to do this?

[tanhauhau]

hmm.. i think the thumbnail currently lives in https://github.com/sveltejs/sites/tree/master/sites/svelte.dev/static/examples/thumbnails

create a jpg file named the same as the url slug name, eg: svelte-element.jpg

[oekazuma]

Thanks!
I will submit a Pull Request with sveltejs/sites

sveltejs/sites#324