azure: Add documentation for the client certs authentication

Added documentation for Azure client certificate authentication.

docs/dev/azure/azure_client_certs_auth.md

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+# Azure Authentication using Client certificates
+
+Azure accepts client certificates as a means of authentication in the service principal and terraform accepts it too. 
+As of 4.12, the Installer accepts certificate-based service principals in addition to secret-based service principals.
+
+### Pitfalls
+Although the installer can now use the certs to authenticate, CCO does not support this and hence the installer
+should create the cluster in manual credentials mode only.
+
+### Prerequisites
+- A certificate that is suitable for Azure is created. 
+- Register the certificate with Azure as part of App Registrations in the Azure AD.
+More information on how to do these steps is below.
+
+## Steps
+The installer takes the service principal for authentication and the current requirement is that these fields are
+populated.
+
+After [1], we no longer need to pass the client secret and can pass the clientCertificate and clientCertificatePassword fields.
+
+1. Populate the service principal with the following fields. The service principal is by default in ~/.azure/osServicePrincipal.json
+-- subscriptionId
+-- tenantId
+-- clientId
+-- clientCertificate (this must be the path to the pfx file generated)
+-- clientCertificatePassword (optional)
+2. Run openshift-installer
+
+The installer will automatically pick up the values in the sevice principal and switch to certificate based authentication.
+
+## Extras
+### Creating a certificate
+Azure expects a PEM file certificate for App registrations that are used for authentication. It then expects the PEM certificate and the
+key to be combined into a PFX file for authentication requests any application makes. We can use openssl to create these certificates.
+There are multiple ways to generate certificate but the key points to remember is that terraform does not accept the latest algorithms 
+for generating pfx files and we need to convert them using old algorithms [2].
+
+To generate a certificate, enter the following command using openssl
+
+`openssl req -x509 -newkey rsa:4096 -keyout key.pem -out cert.pem -sha256 -days 365`
+
+This command will ask a few questions that need to be answered and it generates a PEM certificate and a PEM key file that are valid for one year.
+
+Once this command is done, we need to register this certificate with Azure AD. Navigate to the AD and click on App Registrations.
+We can either reuse an existing registration or create a new one in which case, enter the name and make sure the Supported Account Types is set to
+the appropriate permission. Redirect URI is optional and need not be entered.
+
+Once the registration is created, click on it, navigate to the Certificates and Secrets section and click on the Upload Certificate to upload the PEM
+certificate that we created. This marks the end of the certificate generation section.
+
+We need to now create the pfx file that we would need to authenticate with azure. This can be done with the following command.
+
+`openssl pkcs12 -certpbe PBE-SHA1-3DES -keypbe PBE-SHA1-3DES -export -macalg sha1 -inkey key.pem -in cert.pem -export -out cert.pfx`
+
+This ensures the pfx file is generated with an algorithm that terraform understands. The pfx file is now ready to use for auth and can be set to the
+"clientCertificate" key in the osServicePrincipal.json file mentioned above.
+
+### References
+[1] - PR for enabling Azure certs auth : https://github.com/openshift/installer/pull/6250
+[2] - https://github.com/hashicorp/terraform-provider-azurerm/issues/16228