Caching plugin docs

[i-am-tom]

Description 📝

This PR adds a quickstart guide for the caching plugin. It also renames the current quickstart guide for plugins to be allowlist-specific.

Quick Links 🚀

Assertion Tests 🤖

The documentation now describes how to add server-side caching to an existing Hasura project.

[hasura-bot]

Error testing the assertions. Check the logs.

[hasura-bot]

Redirects 🔀

Hey, @robertjdominguez & @seanparkross — the following redirects will be needed for deployment:

# TEST ME: https://stage.hasura.io/docs/3.0/plugins/quickstart/
location = /docs/3.0/plugins/quickstart/ {
    return 301 https://$host/docs/3.0/plugins/allowlist/;
}

[hasura-bot]

@i-am-tom Thanks for your PR! I've assigned @robertjdominguez to review it.

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[robertjdominguez]

Overall, this is a great addition @i-am-tom 🎉

I made a few suggestions for ordering clarity. One important thing: it was quite clear in @paritosh-08's example for allowlists how this should eventually be deployed. While the docker setup is great for local dev, can we add a section at the end that explains how users should handle referencing the deployed version of this plugin?

From my understanding, we don't host plugins. So, maybe taking the url value for the plugin and throwing it into an env would be a good start? Then, simply explain that in their .env.cloud they'd modify that value to reflect the deployed instance?