Use workflow variables for tool versions in "Check Website" workflow

Placement of this information at the top of the file, where it is easy to find and edit, facilitates updates to the
workflows as the tool version used for project development are bumped periodically.

Author: per1234

.github/workflows/check-mkdocs-task.yml

@@ -1,6 +1,12 @@
 # Source: https://github.com/arduino/tooling-project-assets/blob/main/workflow-templates/check-mkdocs-task.md
 name: Check Website
 
+env:
+  # See: https://github.com/actions/setup-go/tree/v2#readme
+  GO_VERSION: "1.14"
+  # See: https://github.com/actions/setup-python/tree/v2#available-versions-of-python
+  PYTHON_VERSION: "3.9"
+
 # See: https://docs.github.com/en/actions/reference/events-that-trigger-workflows
 on:
   push:
@@ -37,12 +43,12 @@ jobs:
       - name: Install Go
         uses: actions/setup-go@v2
         with:
-          go-version: "1.14"
+          go-version: ${{ env.GO_VERSION }}
 
       - name: Install Python
         uses: actions/setup-python@v2
         with:
-          python-version: "3.9"
+          python-version: ${{ env.PYTHON_VERSION }}
 
       - name: Install Poetry
         run: pip install poetry