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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
Accepting contributions to your paper/code from outside. Github has a convenient mechanism of pull requests – suggestions from other users, that you can then approve and merge into the main content. For example, the Web Development for Beginners course was developed and hosted on GitHub originally by a group of around 10 people, and now it has more than 50 contributors, including people who are translating the course into different languages.
In fact, I've been writing all of my blog posts and most text content in Markdown for a few years, including posts with formulae. For scientific writing, the great Markdown processor (as well as live editing environment) integrated with TeX is madoko – I highly recommend you check it out. You can use it from the web interface (which has GitHub integration), and there's also an open-source command-line tool to convert your Markdown writing into either LaTeX, or directly to PDF.
The first thing you'll want to do is set up Git. Git is the version control system that runs behind the scenes of any GitHub project-- it's what allows you to collaborate with others, go back to previous versions of your project, and view changes made by different members of your team. You may want to use Git from a command-line, but in the beginning, it might be easier to use the GitHub Desktop client.
You can also open the repository in your external editor. I use VS Code to edit my files, so I can add section 2 of my paper there, and then commit it to my section2 branch.