docs: clarify config file creation in Pages quickstart

[royy92]

What

Clarify the GitHub Pages quickstart instructions for editing _config.yml.

Why

Issue #44410 reports that the guide assumes _config.yml already exists. Users who do not have this file in their repository may be unable to follow the instructions and may not realize they need to create the file first.
Closes: #44410

How

• Added a step to check whether _config.yml exists.
• Instructed users to create _config.yml if it is missing before attempting to open and edit it.
• Left the existing workflow unchanged for users who already have the file.

Test

Verified that the updated instructions address the missing _config.yml scenario described in Issue #44410 .

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

[github-actions]

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Table of review links

Note: Please update the URL for your staging server or codespace.

The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

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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