#1959 docs: add 0.18.0 What's New items

Adds two iOS items to the 0.18.0 What's New list and asks future PRs to update the list.

What's New items (0.18.0)

• Faster, smoother gestures (iOS): groups the smoothness work from 0.17.0 and 0.18.0. 0.17.0 had no What's New, so this is the first place users hear about it.
  • 0.18.0: swiping between days (#1939, #1937), the week strip (#1957)
  • 0.17.0: swipe back (#1793), row swipe actions (#1792, #1926), bottom sheets (#1794), image lightbox drag (#1785)
• Sheets with depth (iOS): the drawer depth effect (#1954).

The existing "What's New" item stays first. Desktop still shows only that item.

Docs

• AGENTS.md: a PR that adds a feature or a change users will notice also adds a WHATS_NEW item for the next marketing version.
• CONTRIBUTING.md: one bullet that points to it.

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Summary by Sourcery

Highlight the latest iOS user-facing improvements in the 0.18.0 What's New list and establish guidance for maintaining it.

New Features:

• Add iOS What's New entries highlighting smoother gestures and depth-enhanced sheets for version 0.18.0.

Enhancements:

• Document the expectation that user-visible changes include a corresponding What's New item.

Documentation:

• Update contributor guidance to explain how and when to add What's New items.