The general principle sounds great, and most of the specific ideas listed above are great and good examples of the way we could tackle the issue.
I have to agree with those who removed the mediawiki template in articles, though: those "project X also has info on Y" are generally aimed at providing extra info about the concept itself, detached from its relationship with Wikimedia. In contrast, the Git page and pretty much every documentation page we have here on mw.org are targeted at MediaWiki development and its surrounding ecosystem. After all, this isn't a generic FOSS documentation project, unlike, say, flossmanuals.net. So that kind of outreach seems to be much more suited to disambiguation hatnotes and targeted banners/campaigns, than to that sort of template.
That said, I wholeheartedly agree we ought to invest on those initiatives. We could for instance start a page with the list of MediaWiki/Wikimedia tech related pages, to facilitate marking which already have the appropriate dab headers (and thus provide a quick to-do list for those interested in contributing in this regard). That list would also help pave the way for a banner campaign, so we could use it to start discussing the content of potential banners as well. And it could host the list of ideas above to a more permanent place, where we could further expand it.
Just my 2 cents.