Add swift docc plugin for gh pages docs workflow. #3

	name: Update Docs

	on:
	push:
	branches: [ main ]

	jobs:
	update-docs:
	name: PixarUSD API Swift ${{ matrix.swift }} on ${{ matrix.os }}
	strategy:
	matrix:
	os: [ubuntu-latest]
	swift: ["5.10"]
	runs-on: ${{ matrix.os }}
	steps:
	- uses: swift-actions/setup-swift@v2
	with:
	swift-version: ${{ matrix.swift }}
	- uses: actions/checkout@v2
	- name: Compile Docs
	run: \|
	set -eux
	git config user.email "devs@wabi.foundation"
	git config user.name "furby™"
	git fetch
	git worktree add --checkout gh-pages origin/gh-pages
	export DOCC_JSON_PRETTYPRINT="YES"
	export SWIFTPM_ENABLE_COMMAND_PLUGINS=1
	swift package \
	--allow-writing-to-directory gh-pages/docs \
	generate-documentation \
	--target PixarUSD \
	--disable-indexing \
	--transform-for-static-hosting \
	--output-path gh-pages/docs
	CURRENT_COMMIT_HASH=`git rev-parse --short HEAD`
	cd gh-pages
	# Inject our site at the index
	rm docs/index.html
	rm docs/favicon.ico
	cp index.html docs
	cp favicon.ico docs
	cp CNAME docs
	cp css/main.css docs/css
	cp -R image docs
	git add docs
	# Commit changes
	if [ -n "$(git status --porcelain)" ]; then
	echo "Documentation changes found. Commiting the changes to the 'gh-pages' branch and pushing to origin."
	git commit -m "Update GitHub Pages documentation site to '$CURRENT_COMMIT_HASH'."
	git push origin HEAD:gh-pages
	else
	echo "No documentation changes found."
	fi

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