Add SSH key file specification instructions

[kmanwar89]

Added instructions for manually specifying the SSH key file when connecting to GitHub, including example SSH command and expected output.

Why:

Closes: #42109

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

Details have been added to advise a user to use the -i <filename> command flag while troubleshooting the SSH connection in case they specified a non-default filename. For users that do specify a non-default filename, the SSH standard error (debug1: No more authentication methods to try; [email protected]: Permission denied (publickey).) is ambiguous and provides no guidance on what further troubleshooting steps to take. The edits to this article seek to address this gap.

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.

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Table of review links

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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
authentication/troubleshooting-ssh/error-permission-denied-publickey.md	fpt ghec ghes@ 3.19 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.19 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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