docs: Write up-to-date tutorial for Preload (shaka-project#6555)

This also deletes the old preload design doc, which was confusing users.

Closes shaka-project#6551

docs/tutorials/index.json

@@ -7,6 +7,7 @@
   { "drm-config": { "title": "DRM Configuration" } },
   { "license-server-auth": { "title": "License Server Authentication" } },
   { "license-wrapping": { "title": "License Wrapping" } },
+  { "preload": { "title": "Preloading" } },
   { "ui": { "title": "UI Library" } },
   { "ui-customization": { "title": "Configuring the UI" } },
   { "text-displayer": { "title": "Configuring text displayer" } },

docs/tutorials/preload.md

@@ -0,0 +1,65 @@
+# Preload API
+
+In some situations, you may wish to begin loading an asset before Shaka Player
+has been attached to a video, or before you have fully committed to playing it.
+Perhaps you have a menu in which users select multiple assets, and you want to
+preload the assets they mouse over to reduce load latency when those assets are
+selected.
+In situations like that, the Shaka Player preload API can be useful.
+
+
+#### Basic Usage
+
+To preload an asset, call the preload method on a player instance.
+
+```js
+async function initPlayer() {
+  const player = new shaka.Player();
+  const preloadManager = await player.preload(
+      'https://storage.googleapis.com/shaka-demo-assets/angel-one/dash.mpd');
+  if (preloadManager) {
+    // The asset is something that can be preloaded. Once you have this manager,
+    // you can load it later by passing it to the load method:
+    await player.load(preloadManager);
+    // If you decide to not play the preloaded asset, you can instead destroy
+    // the preload manager:
+    await preloadManager.destroy();
+  } else {
+    // This asset is something that cannot be preloaded (for instance, a raw
+    // media file, or browser-based HLS on Safari), so the promise of the
+    // preload method yielded null.
+  }
+}
+```
+You need a player instance to preload an asset, and you must use the returned
+preloadManager on the same player instance.
+The preloading process loads the manifest and first segments of the asset. It
+will not load the whole asset ahead of time.
+
+The preload method can be provided an optional startTime and mimeType parameter,
+much the same as the load method.
+```js
+const preloadManager = await player.preload(
+    'https://storage.googleapis.com/shaka-demo-assets/angel-one/dash.mpd',
+    15, 'application/dash+xml');
+```
+
+
+#### Saving Preloaders
+
+The preload API can also be used to unload the player while saving some data
+from the playback session, to allow for playback to resume in the future
+without having to download as much.
+To do this, instead of calling unload on the player, call the
+unloadAndSavePreload method:
+
+```js
+// This both unloads the player, and creates a preload manager for the currently
+// playing asset.
+const preloadManager = await player.unloadAndSavePreload();
+```
+
+The resulting preload manager can be used just like one created via the preload
+method. Much like a normal preload manager, it must be used on the same player
+instance that created it. It contains the loaded manifest, and depending on the
+optional configuration values passed in it may contain