added e2e tests (#298)

- added e2e tests for
  - banner
  - history
  - intro
  - language
  - link
- added e2e workflow
- added cypress badge to README

cypress/support/checkLinks.ts

@@ -0,0 +1,15 @@
+// eslint-disable-next-line @typescript-eslint/no-namespace
+declare namespace Cypress {
+  interface Chainable {
+    /**
+     * Custom command to check an external Link.
+     * @example cy.get(a#extern).checkExternalLink('http://example.com')
+     */
+    checkExternalLink (url: string): Chainable<Element>
+  }
+}
+
+Cypress.Commands.add('checkExternalLink', { prevSubject: 'element' }, ($element: JQuery, url: string) => {
+  cy.wrap($element).should('have.attr', 'href', url)
+    .should('have.attr', 'target', '_blank')
+})