[docs-beta] migrate agent-config.md

[cmpadden]

Summary & Motivation

Note that I renamed the file to .mdx since we are using components. I'm not sure what the etiquette is here, so please let me know if that's not correct.

You may find this diff helpful.

$ diff \
    docs/dagster-plus/deployment/management/environment-variables/agent-config.mdx \
    ../content/dagster-plus/managing-deployments/setting-environment-variables-agents.mdx

How I Tested These Changes

Changelog

Insert changelog entry or delete this section.

[github-actions]

Deploy preview for dagster-docs-beta ready!

Preview available at https://dagster-docs-beta-kv1wgrol0-elementl.vercel.app

Direct link to changed pages:

• https://dagster-docs-beta-kv1wgrol0-elementl.vercel.app/dagster-plus/deployment/management/environment-variables/agent-config
• https://dagster-docs-beta-kv1wgrol0-elementl.vercel.app/dagster-plus/deployment/management/environment-variables/agent-config