Document changelog creation via GitLab CLI

What does this MR do?

Thanks to the writeup in cli#1337 (comment 1395388783) by @michael-mead, I have enough information to document how to create a changelog using the GitLab CLI.

I chatted with @garyh and @phikai this morning about the possibility of cutting a new CLI minor release to make this feature available to users. I still think it deserves a release post item in the %16.1 release post, too. (16.0 comes out in 3 days and it'll be full, so waiting would make sense here.)

TBD:

• version number so we can state it in the prerequisites section
• technical writing review, as Amy is the author here

Related issues

• Related to Docs: use glab to generate changelogs (cli#1337 - closed)
• Related to Splits changelog user docs out from API page (!109656 - merged) where I carved the info for the changelogs page out of the API page

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
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  • Style Guide.
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If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

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If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

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