Add authoring support for `@TitleHeading` directive

[ethan-kusters]

Feature Name

@TitleHeading directive

Description

A new @TitleHeading metadata directive for customizing the text of a page-title heading (also known as an eyebrow or kick).

@TitleHeading accepts an unnamed parameter containing containing the page-title’s heading text.

@Metadata {
  @TitleHeading("Release Notes")
}

Motivation

Currently the title heading is automatically populated with the kind of the page. Supporting customization of this value will allow authors to create custom kinds of pages beyond just articles. This will be especially helpful for DocC catalogs containing more book-like content.

Notes

This feature has been pitched on the forums here but its implementation is still pending.

[ethan-kusters]

rdar://110662981