Skip workflows if only non-source-code files are changed (#839)

Skip workflows in PRs if **ONLY** the following are changed:

```
- 'doc/**'
- '*.md'
- '*.json'
- 'README.rst'
- 'LICENSE.txt'
```

Any other changes (especially changes in `pygmt/` and `examples/`) still trigger the workflows.

**Notes:**

- `examples/` directory is not excluded because some examples may pass on one OS but fail on another one.
- the workflow is always triggered in the master branch even though only plaintext files are changed

Fixes #837.

Reference: https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#onpushpull_requestpaths

Author: seisman

.github/workflows/ci_tests.yaml

@@ -7,6 +7,12 @@ on:
   push:
     branches: [ master ]
   pull_request:
+    paths-ignore:
+      - 'doc/**'
+      - '*.md'
+      - '*.json'
+      - 'README.rst'
+      - 'LICENSE.txt'
   release:
     types:
       - published

.github/workflows/ci_tests_dev.yaml

@@ -8,6 +8,12 @@ on:
   #   branches: [ master ]
   pull_request:
     types: [ready_for_review]
+    paths-ignore:
+      - 'doc/**'
+      - '*.md'
+      - '*.json'
+      - 'README.rst'
+      - 'LICENSE.txt'
   repository_dispatch:
     types: [test-gmt-dev-command]
   # Schedule daily tests