Fix GitHub Q&A sample not working with some repos (#2117)

### Motivation and Context

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Address item: #2041

### Description
1. Fill in the required header fields by default.
2. Update ReadMe to include private repo access information.


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### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [ ] The code builds clean without any errors or warnings
- [ ] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#dev-scripts)
raises no violations
- [ ] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄

samples/apps/github-qna-webapp-react/README.md

@@ -36,6 +36,11 @@ only content extracted from markdown files.
 > [!CAUTION]
 > Each function will call Open AI which will use tokens that you will be billed for.
 
+## Working with private repositories
+The GitHub Repo Q&A Bot sample allows you to pull in data from a private GitHub repo. To do so, you must create a [fine-grained personal access token](https://docs.github.com/en/authentication/keeping-your-account-and-data-secure/managing-your-personal-access-tokens#creating-a-fine-grained-personal-access-token) if you have not already. The token must have access to the repository you are trying to pull data from as well as read access to the Contents of the repository:
+![Alt text](contents-read-access.png)
+
+
 ## Next Steps
 
 Create Skills and SK functions: Check out the [documentation](https://aka.ms/sk/learn)

samples/dotnet/KernelHttpServer/Plugins/GitHubPlugin.cs

@@ -98,13 +98,13 @@ public GitHubPlugin(IKernel kernel, ILogger<GitHubPlugin>? logger = null)
             this._logger.LogDebug("Downloading {RepoBundle}", repoBundle);
 
             var headers = new Dictionary<string, string>();
+            headers.Add("X-GitHub-Api-Version", "2022-11-28");
+            headers.Add("Accept", "application/vnd.github+json");
+            headers.Add("User-Agent", "msft-semantic-kernel-sample");
             if (!string.IsNullOrEmpty(patToken))
             {
                 this._logger.LogDebug("Access token detected, adding authorization headers");
                 headers.Add("Authorization", $"Bearer {patToken}");
-                headers.Add("X-GitHub-Api-Version", "2022-11-28");
-                headers.Add("Accept", "application/vnd.github+json");
-                headers.Add("User-Agent", "msft-semantic-kernel-sample");
             }
 
             await this.DownloadToFileAsync(repoBundle, headers, filePath, cancellationToken);