ci: run E2E only on nightly schedule, skip E2E on PR/push/manual CI, keep manual e2e workflow for on-demand runs, and align matrix/docs accordingly

.github/workflows/ci.yml

@@ -343,7 +343,7 @@ jobs:
     name: Detect E2E Suites
     runs-on: ubuntu-latest
     needs: [quality-gate]
-    if: needs.quality-gate.result == 'success'
+    if: always() && needs.quality-gate.result == 'success' && github.event_name == 'schedule'
     timeout-minutes: 10
     outputs:
       matrix: ${{ steps.build-matrix.outputs.matrix }}
@@ -372,7 +372,7 @@ jobs:
 
   e2e:
     needs: [e2e-setup]
-    if: needs.e2e-setup.outputs.matrix != '[]'
+    if: github.event_name == 'schedule' && needs.e2e-setup.outputs.matrix != '[]'
     strategy:
       fail-fast: false
       matrix: