feat: add e2e test suite with Robot Framework

Add e2e/ dir with Robot Framework tests for page loading, ban records,
blocklist import, config edit. Add requirements.txt. Update Makefile with
test commands. Update .gitignore, backend docs, testing requirements docs.

e2e/tests/01_page_loading.robot

@@ -0,0 +1,13 @@
+*** Settings ***
+Resource    ${CURDIR}/../../resources/common.resource
+
+*** Test Cases ***
+Page Loads And Shows Navigation
+    New Browser    chromium    headless=${TRUE}
+    New Page    ${FRONTEND_URL}
+
+    # Confirm the page title or root element is present.
+    ${title}=    Get Title
+    Should Not Be Empty    ${title}
+
+    Close Browser