fix: show index page

[seaerchin]

Problem

index pages display in the sidebar but not in the main view - we should not be showing such index pages for reasons outlined here

Solution

1. update typing using Exclude in router method
2. add filter for IndexPage

Tests

1. add a custom index page in db
2. verify that this cannot be seenin the sidebar

[datadog-opengovsg]

Datadog Report

Branch report: fix/hide-index
Commit report: 8159540
Test service: isomer-studio

✅ 0 Failed, 257 Passed, 36 Skipped, 45.13s Total Time
➡️ Test Sessions change in coverage: 1 no change