Revise quickstart instructions for _config.yml

[vtwebguy]

Updated instructions for editing the _config.yml file to include creating a new file if it doesn't exist.

Why:

Issue 41364 -- directions are slightly wrong for quickstart with pages re: _config.yml

Closes: #41364

What's being changed (if available, include any code snippets, screenshots, or gifs):

Changed steps to account for creating _config.yml instead of updating a non-existant file.

Check off the following:

• [ x] 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.
• [ x] The changes in this PR meet the docs fundamentals that are required for all content.
• [ x] All CI checks are passing and the changes look good in the review environment.

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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@ 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.18 3.17 3.16 3.15 3.14

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

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[Sharra-writes]

@vtwebguy Thanks for the issue and PR! I've just edited the PR a bit to make sure the issue was linking properly. I'll run this by the Pages team to make sure there's nothing they want to add!