Add BDN tests: Operations.RawStringOperations and Operations.CustomOperations

[darrenge]

This PR does the following:

1. Adds BDN tests: Operations.RawStringOperations and Operations.CustomOperations and the corresponding expected values in the config file. Required a minor change in the .ps1 file for the results array (change from size 20 to 70)
2. Adds a new badge (BDN Charts) on to the front of Github.com page (https://github.com/microsoft/garnet)
3. Adds a link to the charts under the "Benchmarking" category on the left (https://microsoft.github.io/garnet/docs/benchmarking/overview)