Also a market for putting a more doc-focused UI on git, integrating that too[1]. Pull Requests are basically gold for doc review, but the process of getting to the PR is something that always seems to need a bit of training. Nothing like the training that's needed to grok the basic graph-based change model, and how it's going to work for natural language (ish) documents, but that's a whole other kettle of fish.
Bonus points for adding a WYSIWYG HTML editor that can work with rendered Markdown and then write its edits back out as Markdown, to Markdown (altho maybe in the worst cases falling back to embedding simple HTML).
[1] GitLens comes pretty close to this, however.