update docs installation guide for install both packages(rehype-pretty-code, shiki)

[whdgur5717]

In the documentation, it currently suggests installing both packages using:

npm install rehype-pretty-code shiki

However, after installing both, I encountered some issues because Shiki v3 changed its API (specifically getSingletonHighlighter)
and in latest version(0.14.0), it is not updated yet.
and rehype-pretty-code relies on dependencies and peerDependencies for a specific version of Shiki, so I think installing both is not necessary.

[atomiks]

Yep, the docs should be shiki@1 for now, since the latest v3 is broken.

Shiki 3 has a PR in progress #257, but needs some attention

Also shiki isn't a dependency, it's a peer deep and is versioned to ^1.3.0. So it should warn that v3 isn't supported (unless installed previously)

[atomiks]

Latest Shiki works now #257