Autodocs (#266)

Automated update the API documentation published to https://aws.github.io/aws-iot-device-sdk-java-v2/

**Issue:**
- Docs weren't updated unless someone remembered to do it manually.
- Manually updating docs whenever you update code pollutes the diff, making it hard to review
- Having HTML docs in `main` branch can cause issues.
   - Bloats size.
   - Generated docs might contain licenses we don't want to be redistributing.

**Description of changes:**
- Docs are kept in `docs` branch now
    - New `docs.yml` Github Workflow updates the `docs` branch whenever the `main` branch changes.
    - New `check-docs` job in `ci.yml` checks pull-requests for anything that would break docs.
    - delete the `docs/` folder in `main` and use `.gitignore` to prevent someone from accidentally resurrecting it.


**TODO:**
Turn on warnings as errors (in mavan-javadoc-plugin configuration add `<failsOnError>true</failsOnError>`). I tried turning it on but it was too much work to tackle today, since a lot of warnings were in the generated code.

Author: graebm

.github/workflows/ci.yml

@@ -4,6 +4,7 @@ on:
   push:
     branches-ignore:
       - 'main'
+      - 'docs'
 
 env:
   BUILDER_VERSION: v0.9.14
@@ -110,4 +111,16 @@ jobs:
       - name: Build ${{ env.PACKAGE_NAME }} + consumers
         run: |
           java -version
-          mvn -B test
+          mvn -B test
+
+  # check that docs can still build
+  check-docs:
+    runs-on: ubuntu-20.04 # latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          submodules: true
+      - name: Check docs
+        run: |
+          mvn install -Dmaven.test.skip
+          ./make-docs.py

.github/workflows/docs.yml

@@ -0,0 +1,40 @@
+# Update the API documentation whenever the `main` branch changes.
+# This documentation lives in its own `docs` branch.
+name: docs
+
+on:
+  push:
+    branches:
+      - 'main'
+
+jobs:
+  update-docs-branch:
+    runs-on: ubuntu-20.04 # latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+        with:
+          submodules: true
+
+      - name: Update docs branch
+        run: |
+          mvn install -Dmaven.test.skip
+          ./make-docs.py
+
+      - name: Commit
+        run: |
+          git config --local user.email "[email protected]"
+          git config --local user.name "GitHub Action"
+          git add --force docs/
+          git commit --message="update docs"
+
+      - name: Push to docs branch
+        uses: ad-m/[email protected]
+        with:
+          github_token: ${{ github.token }}
+          branch: docs
+          # Force push so that `docs` branch always looks like `main`,
+          # but with 1 additional "update docs" commit.
+          # This seems simpler than trying to cleanly merge `main` into
+          # `docs` each time.
+          force: true

.gitignore

@@ -182,3 +182,6 @@ bin/
 .settings
 
 *.log
+
+# docs are updated automatically by .github/workflows/docs.yml
+docs/