Add web documentation

- '/documentation': home page with all links
- '/documentation/handbook': html version of the gm handbook
- '/documentation/gachamapping': json document with the gacha mappings

src/main/java/emu/grasscutter/server/http/documentation/RootRequestHandler.java

@@ -0,0 +1,42 @@
+package emu.grasscutter.server.http.documentation;
+
+import static emu.grasscutter.Configuration.DATA;
+import static emu.grasscutter.utils.Language.translate;
+
+import emu.grasscutter.Grasscutter;
+import emu.grasscutter.data.ResourceLoader;
+import emu.grasscutter.utils.FileUtils;
+import emu.grasscutter.utils.Utils;
+import express.http.Request;
+import express.http.Response;
+import java.io.File;
+import java.nio.charset.StandardCharsets;
+
+final class RootRequestHandler implements DocumentationHandler {
+
+    private final String template;
+
+    public RootRequestHandler() {
+        ResourceLoader.loadResources();
+        final File templateFile = new File(Utils.toFilePath(DATA("documentation/index.html")));
+        if (templateFile.exists()) {
+            template = new String(FileUtils.read(templateFile), StandardCharsets.UTF_8);
+        } else {
+            Grasscutter.getLogger().warn("File does not exist: " + templateFile);
+            template = null;
+        }
+    }
+
+    @Override
+    public void handle(Request request, Response response) {
+        if (template == null) {
+            response.status(500);
+            return;
+        }
+
+        String content = template.replace("{{TITLE}}", translate("documentation.index.title"))
+                .replace("{{ITEM_HANDBOOK}}", translate("documentation.index.handbook"))
+                .replace("{{ITEM_GACHA_MAPPING}}", translate("documentation.index.gacha_mapping"));
+        response.send(content);
+    }
+}