documentation: update gatsby-cli docs to specify process.env.PORT usage

[wardpeet]

Summary

A new option was added to gatsby-cli that makes it possible to use the PORT env variable instead while running the develop command.

The behavior of the develop command hasn't changed. gatsby develop -p 8000 still works and is used over the env variable if provided. With the new changes we can do PORT=4000 gatsby develop and the default port will be 4000. PORT=4000 gatsby develop -p 8080 will still use port 8080 instead of 4000.

Documentation that needs to be updated are:

• https://www.gatsbyjs.org/docs/gatsby-cli/#develop
• https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-cli/README.md

This issue was raised by @muesha #20110 (comment)

Motivation

We need to keep our documentation up to date.

[github-actions]

Hiya!

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