docs: Add markdoc-svelte package

[Greenheart]

markdoc-svelte is a very useful alternative to mdsvex for rendering markdown content. The Markdoc support also makes it easier to create advanced types of content, like composable blocks for website builders and documentation sites.

After successfully using mdsvex for many years, my team has found an even better solution and started using markdoc-svelte in several projects lately. This has enabled much more flexibility for dynamic content, compared to MDSveX and MDX. In addition, markdoc-svelte has better support for layouts, TypeScript, metadata, and Svelte 5 compared to mdsvex.

I hope a low download count is not a blocker for adding new, high quality packages like this one.

I'm not sure about the optimal order, but think it makes sense to showcase early since markdown and similar content types are very common. Perhaps this could even be added as number 2 or 3 to be visible above the fold, since the package is a solid alternative to mdsvex.

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

• It's really useful if your PR references an issue where it is discussed ahead of time.
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.

[vercel]

@Greenheart is attempting to deploy a commit to the Svelte Team on Vercel.

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[benmccann]

I hope a low download count is not a blocker for adding new, high quality packages like this one.

I think that it would be unfortunately. There are just too many packages for us to be able to test and evaluate them all, so we have to rely on a few filters in order to select from a smaller set of packages