Refine content and form for GitHub App credentials

[janfaracik]

Description

Small one to refine the content and form for GitHub App credentials.

• Added a description for the credential type
• Added guidance on how to find your GitHub App info
• Made guidance on converting private key more prominent (this always catches me out)

Before

After

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Before

After

Submitter checklist

• Link to JIRA ticket in description, if appropriate.
• Change is code complete and matches issue description
• Automated tests have been added to exercise the changes
• Reviewer's manual test instructions provided in PR description. See Reviewer's first task below.

Reviewer checklist

• Run the changes and verify that the change matches the issue description
• Reviewed the code
• Verified that the appropriate tests have been written or valid explanation given

Documentation changes

• Link to jenkins.io PR, or an explanation for why no doc changes are needed

Users/aliases to notify