docs: Shift environment variables from unordered list to table

Amy Quallsrequested to merge
7744-aqualls-add-table into main

Description

Based on the conversation in #7744 (closed), changes the key / value list of environment variables in https://gitlab.com/gitlab-org/cli#environment-variables to a three-column table.

I don't have the default values, so I'll need to pair with someone to get those defaults.

Related issues and merge requests

Related to Follow-up from "feat(update check): Add env var... (#7744 - closed)

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