Initial draft GitHub actions (#640)

* Initial creation of firmware build action

* Initial doxygen action for testing

.github/workflows/build-firmware.yml

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+name: Build Firmware
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Cache pip
+      uses: actions/cache@v2
+      with:
+        path: ~/.cache/pip
+        key: ${{ runner.os }}-pip-${{ hashFiles('**/requirements.txt') }}
+        restore-keys: |
+          ${{ runner.os }}-pip-
+    - name: Cache PlatformIO
+      uses: actions/cache@v2
+      with:
+        path: ~/.platformio
+        key: ${{ runner.os }}-${{ hashFiles('**/lockfiles') }}
+
+    - name: Set up Python
+      uses: actions/setup-python@v2
+
+    - name: Install PlatformIO
+      run: |
+        python -m pip install --upgrade pip
+        pip install --upgrade platformio
+
+    - name: Run PlatformIO
+      run: platformio run -e teensy35 -e teensy36 -e teensy41 -e black_F407VE -e megaatmega2561 -e megaatmega2560

.github/workflows/doxygen.yml

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+# This is a basic workflow to help you get started with Actions
+
+name: Doxygen Action
+
+# Controls when the action will run. Triggers the workflow on push or pull request
+# events but only for the master branch
+on:
+  push:
+    branches: [ master ]
+
+
+  
+# A workflow run is made up of one or more jobs that can run sequentially or in parallel
+jobs:
+  # This workflow contains a single job called "build"
+  build:
+    # The type of runner that the job will run on
+    runs-on: ubuntu-latest
+
+    # Steps represent a sequence of tasks that will be executed as part of the job
+    steps:
+    # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
+    - uses: actions/checkout@v2
+        
+    - name: Doxygen Action
+      uses: mattnotmitt/doxygen-action@v1.1.0
+      with:
+        # Path to Doxyfile
+        doxyfile-path: "./Doxyfile" # default is ./Doxyfile
+        # Working directory
+        working-directory: "." # default is .