Auto-label PRs based on their content (rapidsai#691)

This PR adds the GitHub action [PR Labeler](https://github.com/actions/labeler) to auto-label PRs based on their content. 

Labeling is managed with a configuration file `.github/labeler.yml` using the following [options](https://github.com/actions/labeler#usage).

**Note: ** This is a duplicate of rapidsai#681 that we're re-merging since it was merged incorrectly.

Authors:
  - AJ Schmidt (@ajschmidt8)
  - Joseph (@jolorunyomi)

Approvers:
  - Joseph (@jolorunyomi)

URL: rapidsai#691

Author: ajschmidt8

.github/labeler.yml

@@ -4,22 +4,18 @@
 
 Python:
   - 'python/**'
-  - 'notebooks/**'
-  
-librmm:
-  - 'cpp/**'
 
 CMake:
   - '**/CMakeLists.txt'
   - '**/cmake/**'
-  
-Java:
-  - 'java/**'
-  
-Ops:
-  - '.github/**'
-  - 'ci/**'
+
+conda:
   - 'conda/**'
-  - '**/Dockerfile'
-  - '**/.dockerignore'
-  - 'docker/**'
+
+cpp:
+  - 'include/**'
+  - 'tests/**'
+  - 'doxygen/**'
+
+gpuCI:
+  - 'ci/**'