Add gh action to build documentation site

Related lunarmodules#332.

Add doc-site target to make file and yml file to build the docs.
Following domenic's guide [1] but using leafo's gh actions like
luacheck.yml.

Builds the documentation for ldoc using the checked out version of ldoc
and uses peaceiris/actions-gh-pages to publish.

Once merged, docs will be available on https://lunarmodules.github.io/LDoc

[1]: https://gist.github.com/domenic/ec8b0fc8ab45f39403dd

.github/workflows/doc.yml

@@ -0,0 +1,32 @@
+# Based on https://gist.github.com/domenic/ec8b0fc8ab45f39403dd
+name: Build Docs
+on:
+  pull_request:
+    branches:
+    - master
+  push:
+    branches:
+    - master
+jobs:
+  build:
+    name: Build docs
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+    - name: Setup Lua
+      uses: leafo/gh-actions-lua@v8
+      with:
+        luaVersion: 5.4
+    - name: Setup Lua Rocks
+      uses: leafo/gh-actions-luarocks@v4
+    - name: Setup dependencies
+      run: luarocks install --only-deps https://raw.githubusercontent.com/lunarmodules/LDoc/master/ldoc-scm-3.rockspec
+    - name: Build docs
+      run: make doc-site
+    - name: Deploy
+      if: ${{ github.event_name == 'push' && github.ref == 'refs/heads/master' }}
+      uses: peaceiris/actions-gh-pages@v3
+      with:
+        github_token: ${{ secrets.GITHUB_TOKEN }}
+        publish_dir: ./out

Makefile

@@ -49,6 +49,9 @@ test-tables:
 
 test-clean: clean-basic clean-example clean-md clean-tables
 
+doc-site:
+	cd $(_REPODIR)/doc && lua $(_REPODIR)/ldoc.lua .
+
 CLEAN=&& lua $(_REPODIR)/ldoc.lua . && rd /S /Q cdocs && cp -rf doc cdocs
 
 clean-basic: