docs(pages): clarify when to create _config.yml

[happysnaker]

Summary

• clarify that the Pages quickstart may not already contain a _config.yml file
• explain how to create _config.yml when it doesn't exist yet
• keep the example focused on adding theme, title, and description

Closes #44410

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Source	Review	Production	What Changed
pages/quickstart.md	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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[happysnaker]

Closing this in favor of the earlier open PR #44821, which covers the same issue. I opened this before noticing the existing contribution and want to avoid duplicate review load on maintainers.