examples: drop "with Shiki", add "with Prism"

[mrienstra]

Note: this issue proposes code changes to withastro/astro/examples, but the only way to make a new issue on withastro/astro is to open a "Bug Report", so here seems like the next best option, as it arguably falls under the umbrella of "documentation".

Note: "examples" are also referred to as "templates" or "official example projects", and are shown at https://astro.new/

Given that Shiki is now the default for syntax highlighting (see Markdown & MDX > Syntax Highlighting):

The "Markdown Shiki" integration example no longer makes sense. Seems like it should be replaced with a new (but very very similar) "Markdown Prism" integration example.

For reference, the with-markdown-shiki example was added in withastro/astro#2497, which was part of RFC 4 "Replace Prism with Shiki".

[sarah11918]

Hi @mrienstra! This isn't really a docs issue at all, but it is good feedback! (For reference, I think it would have been fine to open this as a bug report and let the team triage it. Or, we do have a #feedback channel in our Discord where people will often bring up issues like this that are more "feedback" in nature!)

I will mention this to @bholmesdev and other core members who may decide to remove this template entirely (as we've recently done with a couple of templates that seemed similarly less useful now). I'm not sure whether they will consider a Prism template useful to add in its place, but I do think it at least makes sense to decide whether this Shiki template is still useful.

[bholmesdev]

Thanks @sarah11918! Just discussed with core, and it sounds like a with-prism would be pretty helpful as a link from our docs. I'm in favor of creating a with-prism!