remark-slug and remark-external-links are deprecated

[JReinhold]

We use two remark plugins to support our MDX functionality in addon-docs and builder-vite:

1. remark-slug
2. remark-external-links

Both of these plugins are marked as deprecated and the maintainer (who also maintains MDX) advices to switch to their rehype equivalent instead, rehype-slug and rehype-external-links.

If it doesn't produce any breaking changes, we should switch them over.

Additional context

remark-slug is used to automaticall add id to headings, which will create the link functionality to headings. That functionality should be preserved.

remark-external-links adds target="_blank" to all external links.

[MehakFatima24]

Hi, this is my first time contributing, is it okay if I take up this issue?

[MehakFatima24]

@JReinhold I have fixed the test cases, it would be extremely kind of you to take a look at this MR whenever its convenient. Dankjewel!

[chakAs3]

I'm checking your PR @MehakFatima24 doing some tests. normally should work it is been legacy for 2 years now and they advice to switch to rehype