Can we learn SEO here, on Github or it’s just a Hub of Programmers?
You’ll need to search for the “SEO” keyword on GitHub and see whether you find any documents, books, courses, etc.
No, many people use version control (and hence, Git and GitHub) to edit documents too. So you’ll find many repositories containing book sources, e.g. in AsciiDoc, markdown, or some other text-based lightweight markup syntax which can be edited collaboratively via Git.
Version control is not limited to programming only, so you’ll find repos for documents, websites, data-sets, and all sorts of things — as long as the contents are plaintext files, Git can track their versioning, so chances are that you’ll see a repository for that use case.