Create Classes section

- create classes/index.md
- create main classes page _includes/classes.md
- create class page _includes/classes/SOA.md
- Add FB, Github, schedule links to SOA.md
Created section for Classes and Instruction

- created main /classes subsection and /classes/SOA
- added sidemenu option for classes
- added SOA.md: uses information from concentration description
  and online links and resources
Start section on Classes and Instruction