DOC-1069 Adds the ability to set announcements on the pages too

[Deflaimun]

Adds the support to make announcements for specific pages/products/site.

Usage

To add a page-specific announcement add these keys in your page:

:page-announcement: true
:page-announcement-text: Page-specific announcement
:page-announcement-link: 'Learn more.'
:page-announcement-link-text: 'http://mylink.here.com'

To add a product-wide announcement, add those keys in the corresponding antora.yaml file:

asciidoc:
  attributes:
    keys:
      announcement: true
      announcement-text: 'product-wide test'
      announcement-link-text: 'Learn more.'
      announcement-link: 'http://mylink.here.com'

To add a site-wide announcement, continue adding the keys in the corresponding site yaml (playbook). Example with local-antora-playbook.yaml:

site:
  keys:
    announcement: true
    announcement-text: 'Site-wide announcement.'
    announcement-link-text: 'Learn more.'
    announcement-link: 'http://mylink.here.com'

The implementation respects the following order:
page > product > site

PS.
The file announcement-bar was auto-idented, that's why the changes look bigger than they are.

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[Deflaimun]

Latest change enables page > product > site announcements

[JakeSCahill]

We might need unique keys for different announcement types. For example, if both site-wide and page-wide is enabled, and I close site-wide, page-wide should stay open