fix: allow custom titles in index pages

[dcshzj]

Problem

The generated index pages do not follow the title of the folder resource, which results in the need to have custom index pages.

Solution

Breaking Changes

• Yes - this PR contains breaking changes
• No - this PR is backwards compatible

Bug Fixes:

• Take the title of the folder resource as the page title when auto-generating the index page.
  • This also allows the site editor to customise the casing of the title without the need to create a custom index page.

Before & After Screenshots

Custom title (permalink: about-us, title: About USSS)

Custom index page (permalink: csa, title: Index)

Tests

• Create a new folder in Studio with a custom title (using random casing) and a permalink with alphabets that are different from the title.
• Create a new page inside this new folder and publish it.
• Verify on the built staging site that the folder that you created shows the custom title that you input earlier.

[datadog-opengovsg]

Datadog Report

Branch report: fix/index-custom-title
Commit report: 9189f3a
Test service: isomer-studio

✅ 0 Failed, 257 Passed, 36 Skipped, 50.37s Total Time
➡️ Test Sessions change in coverage: 1 no change