docs: fix links in zero-installs explanation

[bitjson]

What's the problem this PR addresses?

I spent a long time debugging a failing yarn install --immutable --immutable-cache --check-cache in my CI environment.

I was seeing the error YN0056: PACKAGE@npm:x.x.x: Cache entry required but missing for PACKAGE@npm:x.x.x for every package in the cache. (The issue was clearly not just for packages with native dependencies like in #4514.)

After trying in multiple environments, I finally realized .pnp.cjs was changing based on the nesting depth of the repo, as enableGlobalCache was not set to false in .yarnrc.yml. (This occurred after an upgrade from Yarn v3 to Yarn v4.)

(Including the above to help anyone searching for this issue on GitHub.)

How did you fix it?

This PR fixes the broken PnP link and makes the offline mirror text a link as well. Hopefully that will help others to realize that to get Zero Installs working they'll need to read that section too.

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.

[SalahAdDin]

Mine does not use pnp.