docs: Append yarn-plugin-pin-deps to Contrib plugins

[milesrichardson]

What's the problem this PR addresses?
This PR adds a new plugin to the list of "contrib plugins". The plugin is yarn-plugin-pin-deps, which adds a command yarn pin-deps. This command will find any dependencies referenced with a semver identifier, and will update package.json to replace that identifier with the exact version of the package currently resolved in the lockfile for that reference.

We've been using it "in production" for 1+ year and I recently migrated it to support Yarn 3, so I decided to also take the opportunity to package it as a plugin. It's worked well for our purposes, where the original goal was to migrate a mono-repo to use pinned dependencies. By it's nature it isn't something we use often, but we do occasionally use it to pin some dependency we accidentally installed without -E a few commits prior (granted, if I didn't have this plugin, I'd just remove the package and install it again - but it does help for migrating many dependencies all at once).

It's possible this plugin is not even necessary, or there is a way to accomplish this dependency pinning with basic yarn commands, but I'm not aware of it (at least, it was certainly not possible when I originally wrote this plugin). It's also possible that is has some oversight in how it pins dependencies that will cause weird bugs, but if so, I have not encountered it yet.

Please do let me know if there is better-supported way to pin dependencies to their currently resolved versions, so that I can delete the plugin 😄

How did you fix it?
I followed the "open a PR" link and used the wonderful GitHub editor to append a link to the plugin 😄 I hope I did not break any formatting.

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.

[arcanis]

Please do let me know if there is better-supported way to pin dependencies to their currently resolved versions, so that I can delete the plugin 😄

Something I wanted to allow was to have resolutions available within the constraints runtime. This way you could just have a constraint define that all dependencies must have their resolutions as range, and it'd auto-fix everything 🤔

[merceyz]

but we do occasionally use it to pin some dependency we accidentally installed without -E a few commits prior

You can use defaultSemverRangePrefix to make yarn add always pin the dependency.

defaultSemverRangePrefix: ''