Docs: add details about the CI environment variable available to gatsby build for dumb terminals

[brettinternet]

Description

Some dumb terminals print to the console excessively with Gatsby's build output, printing a new line for each animation.

While ci-info does detect many CI/CD environments, it may be necessary to explicitly declare CI=true to make ci-info aware of its CI environment. As far as I understand, ci-info's module isCI conditionally determines whether ink will be used to update build activity to the console.

In addition to CI, the multi-core build environment variable is relevant to document alongside since both are likely to be useful in virtual integration environments, as I found in constructing my own build pipeline with Gatsby. Some connection should be made from gatsby build to the available environmental variable side effects.

Related Issues

Addresses #17873

[sidharthachatterjee]

This is great! Thank you so much 🥇

[gatsbot]

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