Add convergent formulation documentation

[zfergus]

Description

• Add documentation describing the convergent formulation
• Add documentation describing the constraint offset/minimum distance
• Add documentation for broad- and narrow-phase CCD
• Add documentation for High-Order IPC
• Add bibtex for citations
• Rename CollisionConstraint::minimum_distance $\rightarrow$ CollisionConstraint::dmin

Checklist:

• I have followed the project style guide
• My code follows the clang-format style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules