Refine instructions for creating a branch via the branches overview

[ashis-baral]

Why:

fix: #45780

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

Updated the "Creating a branch" instructions to:

• Change "Branch name" to "New branch name" to match the current UI.
• Change "Branch source" to "Source".
• Remove the reference to choosing a repository, since the current branch creation dialog does not provide a separate repository selection.
• Change "Create branch" to "Create new branch" to match the current button label.

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The updated steps are:

1. Under "New branch name", type a name for the branch.
2. Under "Source", choose the branch to base your new branch on.
3. Click Create new branch.

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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
pull-requests/how-tos/commit-changes/managing-branches-within-your-repository.md	fpt ghec ghes@ 3.22 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.22 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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[subatoi]

Thanks @ashis-baral—this is much appreciated, again. I'll get this merged now 👍

[github-actions]

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