Consolidate content contribution guidelines

The following discussion from !3534 (merged) should be addressed:

• @pedroms started a discussion: (+2 comments)

  @jeldergl @danmh thanks for this helpful content. When looking at this from the point-of-view of a contributor, I see an opportunity to consolidate the guidelines when contributing content to Pajamas. I found the following places with content writing guidelines:

  1. Documentation MR template: links to helpful content, but only if it's a new page:

     • If creating a new component page from scratch, it follows the page template structure.
     • Content follows the Pajamas voice and tone guidelines, falling back on the GitLab Documentation Style Guide when needed.

  2. Page template: lots of useful guidelines and tips, particularly at the top, but again, only seen if it's a new page.

  3. README: two sections with related content (Writing component documentation and Adding graphics and images), but not shown consecutively, and under the “Development” parent section.

  Finally, none of these touchpoints link to the Adding graphics and images section of the README.