Cut 1.39's heading and write 1.40's store copy

[andiwand]

Everything under Unreleased has landed, so it is a version: ## [Unreleased] becomes ## [1.40], and fastlane/metadata/en-US/changelogs/1.40.txt holds the "What's New" copy for it.

What 1.40 is:

#155	the tool bar sat on the status bar, with an empty band between its buttons and the document
#156	a back chevron instead of "Back to documents", and a pencil for the documents that can be edited
#157	a document opens fitted to the screen instead of running off the edge, as it does on Android

No date and no compare link on the heading, and [Unreleased] stays on v1.39...HEAD: the tag is written when the drafted release is published, which is what CHANGELOG.md says to wait for.

Both release scripts were run against this branch, since they are what a dispatch would hit first:

$ .github/scripts/resolve-version.py --input 1.40 --dry-run false
building 1.40

$ .github/scripts/changelog-section.py --version 1.40
### Changed
- A document wider than the screen opens fitted to it rather than running off
  the edge, which is what it has always done on Android.
…

That section is what the GitHub release body becomes.

Once this is merged: gh workflow run release.yml -f version=1.40.

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