Byline: adding documentation, moving to main title

[Garneauma]

• Adding byline documentation so that it can be deprecated
• Adding "Main page title with byline" variation to the Main page title (this variation is limited to news pages on Canada.ca)

Related to WET-494

[Garneauma]

@duboisp I have set the status of the byline component to deprecated. However, if we want to get this pull request out without triggering a major release, I can change it to demoted instead.

[Garneauma]

Pre-approved upon successful review.

[duboisp]

See my comment inline.

Note: We will need to discuss if we apply that by default to the last paragraph in the hgroup or if we require this to be activated by the author by using a CSS class. The second option will allow us to use the paragraph under the h1 for something else considering that the trial currently is to remove the byline.