Dumping thoughts relate to expanding https://www.mediawiki.org/wiki/Documentation and subpages, such as https://www.mediawiki.org/wiki/Documentation/Style_guide#Structuring_pages . See also T277128: Improve mw:Documentation as a landing page for technical documentation writers.

About page creation itself:

• Before creating a page, first search for existing pages to potentially expand - cf https://meta.wikimedia.org/wiki/Learning_patterns/Every_new_page_starts_off_unwatched
• Before creating a page, first search for existing top-level pages (e.g. mw:Phabricator) to avoid creating new top-level pages (e.g. mw:Phabricator_aspect); encourage creation of subpages instead (mw:Phabricator/Aspect))
• After creating a page, check Special:WhatLinksHere in the sidebar and make sure that the page is linked from relevant pages, e.g. a top page, or (and that is harder to find and identify) any related "link hub style" templates (random example, as I lack better words: meta:Template:Communications

About technology and people:

• I may feel intimidated by module / template / <translate> stuff on pages (cf. T131516: Reduce or eliminate the need for the user to touch <translate> tags and unit markers) that I want to edit. Creating a new page with partial duplication, instead of potentially breaking something that I don't fully understand, can be easier and more comfortable.
• People take paths of least residence: I'd rather copy a theme instead of creating my own. But that requires me to find a theme first, and the theme to copy should be 'clean'.

Which leads to having a "library" of page structure recommendations / elements for common use cases / patterns. Boilerplates. Random examples:

• The "Documentation" section for new extension codebases at mw:Writing_an_extension_for_deployment - covers e.g. what to use Extension:Foo and Help:Extension:Foo for, doesn't cover Manual:... (also cf. https://meta.wikimedia.org/wiki/Learning_patterns/Writing_a_new_MediaWiki_extension_for_deployment_on_a_Wikimedia_project )
• WMF Engineering Team pages often/always (?) seem to use {{Wikimedia engineering project information}} (but that's only a box template; not content structure)
• Content structure: Stuff like T115853: Every WMF engineering team should have a "Contact" section on its public wiki page
• As naming things is hard, any template/style recommended to "distinguish" at the top? (Example: Improvised first three lines on mw:Community_metrics)