Adding --check to run a serials network test against GitHub or GHES. (#900)

* add --check.

docs/checks/network.md

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+## Common Network Related Issues
+
+### Common things that can cause the runner to not working properly
+
+- Bug in the runner or the dotnet framework that causes actions runner can't make Http request in a certain network environment.
+
+- Proxy/Firewall block certain HTTP method, like it block all POST and PUT calls which the runner will use to upload logs.
+
+- Proxy/Firewall only allows requests with certain user-agent to pass through and the actions runner user-agent is not in the allow list.
+
+- Proxy try to decrypt and exam HTTPS traffic for security purpose but cause the actions-runner to fail to finish SSL handshake due to the lack of trusting proxy's CA.
+
+- Firewall rules that block action runner from accessing certain hosts, ex: `*.github.com`, `*.actions.githubusercontent.com`, etc.
+
+
+### Identify and solve these problems
+
+The key is to figure out where is the problem, the network environment, or the actions runner?
+
+Use a 3rd party tool to make the same requests as the runner did would be a good start point.
+
+- Use `nslookup` to check DNS
+- Use `ping` to check Ping
+- Use `curl -v` to check the network stack, good for verifying default certificate/proxy settings.
+- Use `Invoke-WebRequest` from `pwsh` (`PowerShell Core`) to check the dotnet network stack, good for verifying bugs in the dotnet framework.
+
+If the 3rd party tool is also experiencing the same error as the runner does, then you might want to contact your network administrator for help.
+
+Otherwise, contact GitHub customer support or log an issue at https://github.com/actions/runner