docs(ui-tests): add minimal docs for testing

Author: phanirithvij

docs/manuals/contributor/how_to/test.md

@@ -0,0 +1,26 @@
+# How to test
+
+## How to test the UI
+
+Run tests using [Playwright](https://playwright.dev/):
+
+```bash
+dev-ui-mock
+
+playwright test -c ui/tests/e2e
+playwright test -c ui/tests/e2e --project=chromium
+playwright test -c ui/tests/e2e --project=mobile
+```
+
+```
+playwright test -c ui/tests/e2e --ui
+playwright test -c ui/tests/e2e --ui-host 127.0.0.1
+```
+
+```
+env BASE_URL="https://ngi-nix.github.io/forge/" playwright test -c ui/tests/e2e --ui-host 127.0.0.1
+```
+
+### Notes
+
+- Use `data-testid` in Elm for stable selectors when writing tests.