UI/UX Markdown Quick Start doc

[gideonthomas]

Allow creating a markdown file to describe instructions for creating web apps which maybe are rendered instead of the default index.html file.

[humphd]

Depends on #238 for backend bits.

My idea here is that we introduce conventions around having a markdown file in a multi-file project, and if present, render it along with the preview. For example, imagine if README.md, INSTRUCTIONS.md, LEARN.md, etc were names that triggered the editor to do something different on start-up. Normally we look for an index.html file to render in the preview. However, we could also choose to show a markdown file if present, and/or provide a switchable preview view (maybe tabs?) that let you jump between the preview HTML and the markdown.

This would let us easily support having instructions/tutorials written in markdown inside a project.

[humphd]

This is part of what we're discussing for tutorials in MVP, cc @flukeout.

[humphd]

I've basically got this working in #392 for a tutorial.html file. It's unclear whether or not we want to support Markdown or not, based on conversations with Hannah and Luke. I might rip the MD stuff out.

[humphd]

This is landed.