Add documentation for actions/setup-node pnpm cache

[jacobwgillespie]

Why:

The actions/setup-node action now supports pnpm caching as of v2.3.0, and @MaksimZhukov requested a PR to update the docs for that addition.

What's being changed:

I've added the workflow YAML example from https://github.com/actions/setup-node#caching-packages-dependencies for pnpm underneath the existing npm and yarn examples, matching the order from the action README.

Check off the following:

• I have reviewed my changes in staging (look for the latest deployment event in your pull request's timeline, then click View deployment).
• For content changes, I have completed the self-review checklist.

Writer impact (This section is for GitHub staff members only):

• This pull request impacts the contribution experience
  • I have added the 'writer impact' label
  • I have added a description and/or a video demo of the changes below (e.g. a "before and after video")

[welcome]

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[MaksimZhukov]

Looks good to me, thank you! Nitpick: I think it make sense to mention the pnpm package manager in the Caching dependencies to speed up workflows guide.

[jacobwgillespie]

I think it make sense to mention the pnpm package manager in the Caching dependencies to speed up workflows guide.

Added! 👍

[ramyaparimi]

@jacobwgillespie Thanks so much for opening a PR! I'll get this triaged for review ⚡

[skedwards88]

Thanks for adding this! I'm going to convert the disclaimer to a reusable, then I'll get this merged down for you.

[github-actions]

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