Convert some sections to markdown format so including markdown files will render correctly

Change some of the chapters that include markdown files to markdown format so they will render as markdown rather than as literals.

Introduction
QuickStart
Scientific Validation
Testing (although the inclusion of the PF Unit README possibly should stay as literal?
Creating Surface Datasets

### What sort(s) of documentation issue is this?
- [ ] Something is missing.
- [x] Something is (or might be) incorrect or outdated.
- [ ] Something is confusing.
- [ ] Something is broken.

### What part(s) of the documentation does this concern?
- [ ] [Technical Note](https://escomp.github.io/CTSM/tech_note/index.html) (science and design of the model)
- [x] [User's Guide](https://escomp.github.io/CTSM/users_guide/index.html) (using the model and related tools)
- [ ] Somewhere else (e.g., README file, tool help text, or code comment): _Please specify_
- [ ] I don't know

### Describe the issue
We include README markdown files in the User's Guide so that we can document things in one place and not have to update both the README files AND the User's Guide. This is a good thing to keep doing moving forward. But, in most cases we want the Markdown files to render well so it has the proper formatting of titles. There may be some cases where