Add a section to minor/major release notes to highlight significant updates

[emilyanndavis]

To help users become aware of new features and other significant changes in a new release of InVEST, we'll add a short section to the top of the release notes that summarizes a few updates we think are worth emphasizing. This section should be included for minor and major releases only (not bugfix releases).

Inspired by the "In a nutshell" section in GDAL Release Notes.

This task will be considered done when HISTORY.rst has been updated with

1. initial content for the upcoming release of 3.15.0 and
2. comments to support implementation in future minor/major releases.

Some related changes may also be required, e.g., to ensure the in-workbench changelog code is compatible with HISTORY's updated structure.