docs: add activeOrganizationId on org schema

demo/nextjs/lib/plugin.ts

@@ -0,0 +1,40 @@
+import { BetterAuthPlugin } from "better-auth";
+
+export const addAccountToSession = {
+	id: "add-account-to-session",
+	hooks: {
+		after: [
+			{
+				matcher(context) {
+					return context.path.startsWith("/callback");
+				},
+				async handler(ctx) {
+					const sessionCookie = ctx.responseHeader.get(
+						ctx.context.authCookies.sessionToken.name,
+					);
+					if (!sessionCookie) {
+						return;
+					}
+					const provider = ctx.path.split("/callback")[1];
+					if (!provider) {
+						return;
+					}
+					const sessionId = sessionCookie.split(".")[0];
+					await ctx.context.internalAdapter.updateSession(sessionId, {
+						accountId: provider,
+					});
+				},
+			},
+		],
+	},
+	schema: {
+		session: {
+			fields: {
+				accountId: {
+					type: "string",
+					required: false,
+				},
+			},
+		},
+	},
+} satisfies BetterAuthPlugin;