Separate Markdown Style Guidelines from General Guidelines

# 💡 Summary #

Break out all of the Markdown specific guidelines in `style-guide.md` into an appropriate `languages/markdown` section.

## Motivation and context ##

Currently the information in `style-guide.md` contains plain language guidelines intermixed with Markdown specific guidelines. I believe it would be better served to break out any Markdown specific guidelines into it's own language section. This will make it more obvious where to find the information as well as improving maintainability.

## Implementation notes ##

The PR should be limited to creating a new `languages/markdown` section and only moving existing Markdown content in `style-guide.md` to this section. Any additions/modifications to the Markdown guidelines should be kept to their own PRs after this separation.

## Acceptance criteria ##

- [ ] Create the PR
- [ ] Everyone approves the PR