docs: use sapper/cypress for e2e testing

docs/cypress/integration/Button.js

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+describe("Button", () => {
+  beforeEach(() => {
+    cy.examples("Button");
+  });
+
+  it("renders correctly", () => {
+    cy.get("button").then(($) => {
+      expect($.length).to.eql(4);
+    });
+  });
+});

docs/cypress/plugins/index.js

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+/// <reference types="cypress" />
+// ***********************************************************
+// This example plugins/index.js can be used to load plugins
+//
+// You can change the location of this file or turn off loading
+// the plugins file with the 'pluginsFile' configuration option.
+//
+// You can read more here:
+// https://on.cypress.io/plugins-guide
+// ***********************************************************
+
+// This function is called when a project is opened or re-opened (e.g. due to
+// the project's config changing)
+
+/**
+ * @type {Cypress.PluginConfig}
+ */
+module.exports = (on, config) => {
+  // `on` is used to hook into various events Cypress emits
+  // `config` is the resolved Cypress config
+}

docs/cypress/support/commands.js

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+Cypress.Commands.add("examples", (component) => {
+  cy.visit(`/examples/${component}`);
+});

docs/cypress/support/index.js

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+import "./commands";