Migrate to gitbook (#357)

This PR facilities the migration from the old Writerside documentation
platform to Gitbook.

## motivation

Gitbook supports serving multiple versions of the documentation, e.g.
v1.12.1, v1.13.0, and master.

web/src/routes/dashboard/about/+page.svelte

@@ -46,7 +46,7 @@
 		explore trending content—all in one place.
 	</p>
 	<p class="mb-2 text-sm text-muted-foreground">
-		Version: v{PUBLIC_VERSION}
+		Version: {PUBLIC_VERSION}
 	</p>
 	<h2
 		class="mt-10 scroll-m-20 pb-2 text-3xl font-semibold tracking-tight transition-colors first:mt-0"

web/src/routes/login/+layout.svelte

@@ -31,7 +31,7 @@
 			<a
 				target="_blank"
 				class="underline"
-				href="https://maxdorninger.github.io/MediaManager/troubleshooting.html"
+				href="https://maximilian-dorninger.gitbook.io/mediamanager/troubleshooting"
 			>
 				Trouble logging in?
 			</a>