Class material unification

This initiates the amalgamation of slides, presenter notes, lab objectives, and workbook materials into unified pages. One markdown file will represent the entire content for one class (*Foundations*, *Intermediate*, *Advanced).

**Objectives pertinent to this PR specifically**
- [x] Build layout for easy curriculum page browsing
- [x] Add TOC and scroll spy support
- [ ] Design stylesheet for curriculum page

**Objectives to migrate to content migration PR (TBD)**
- [ ] Migrate all separate markdown [slides files](https://github.com/github/training-kit/tree/master/slides/_posts) into the appropriate [curriculum](https://github.com/github/training-kit/tree/class-material-unification/curriculum) markdown file
- [ ] Move all [workbook](https://github.com/github/training-kit/tree/master/workbooks) content into "Lab" sections of curriculum
- [ ] Deprecate the [slides](https://github.com/github/training-kit/tree/master/slides) and [workbooks](https://github.com/github/training-kit/tree/master/workbooks)