Polish Documentation

[amageh]

I am using this issue to collect to-do's for the documentation. The reference branch is docs2

Content

• Add article on sparsity conditions
• Add guide on maximal example similar to simple model
• Add solution benchmarks article (or remove for now)
• Add research projects and theses if available.
• Add (short) roadmap and contact info
• Add more information to main page/index page.
• Update Two period model and interface functions guide to new interface.

Minor issues

• Fix issue with math rendering in Jupyter notebooks
• Make sure dcegm is written consistently across the documentation (current version contains: dc-egm, dcegm, and DCEGM)
• Review exogenous processes guide for new interface (i.e. change wording to stochastic states).
• Fix dependencies so notebooks run properly in documentation.