Add docc support and publish documentation to GitHub pages (apple#137)

* Move Guides into docc base location

* Add Github gh-page support

* Conditionally add docc-plugin dependency

* Temp skip AsyncSequenceValidation target

Author: Kyle-Ye

Package.swift

@@ -41,3 +41,18 @@ let package = Package(
       ]),
   ]
 )
+
+#if canImport(Darwin)
+import Darwin
+let buildingDocs = getenv("BUILDING_FOR_DOCUMENTATION_GENERATION") != nil
+#elseif canImport(Glibc)
+import Glibc
+let buildingDocs = getenv("BUILDING_FOR_DOCUMENTATION_GENERATION") != nil
+#else
+let buildingDocs = false
+#endif
+
+// Only require the docc plugin when building documentation
+package.dependencies += buildingDocs ? [
+  .package(url: "https://github.com/apple/swift-docc-plugin", from: "1.0.0"),
+] : []

Sources/AsyncAlgorithms/AsyncAlgorithms.docc/AsyncAlgorithms.md

@@ -0,0 +1,36 @@
+# ``AsyncAlgorithms``
+
+**Swift Async Algorithms** is an open-source package of asynchronous sequence and advanced algorithms that involve concurrency, along with their related types.
+
+## Overview
+
+This package has three main goals:
+
+- First-class integration with `async/await`
+- Provide a home for time-based algorithms
+- Be cross-platform and open source
+
+## Topics
+
+### Getting Started
+
+- <doc:AdjacentPairs>
+- <doc:BufferedBytes>
+- <doc:Chain>
+- <doc:Channel>
+- <doc:Chunked>
+- <doc:Collections>
+- <doc:CombineLatest>
+- <doc:Compacted>
+- <doc:Debounce>
+- <doc:Effects>
+- <doc:Intersperse>
+- <doc:Joined>
+- <doc:Lazy>
+- <doc:Merge>
+- <doc:Reductions>
+- <doc:RemoveDuplicates>
+- <doc:Select>
+- <doc:Throttle>
+- <doc:Timer>
+- <doc:Zip>