Writie initial Getting Started documentation

[nickrobinson251]

We currently have some deployed docs at http://www.juliadiff.org/ChainRules.jl/latest/
crying out for "getting started" documentation.

Currently 4 sections are suggested

• Forward-Mode vs. Reverse-Mode Chain Rule Evaluation
• Real Scalar Differentiation Rules
• Complex Scalar Differentiation Rules
• Non-Scalar Differentiation Rules
  ...but some other categorisation might work fine.

Adding even one section would be a great start.

Lots of things have docstrings now... but adding more would also be helpful!

[aman-bhu]

Hey, I want to contribute, can you help me how to get started?

[oxinabox]

Some links which should be read to form basic of this getting started documentation:

• source code of https://github.com/JuliaDiff/ChainRulesCore.jl
• The source for the micro-ADs linked in https://github.com/JuliaDiff/ChainRules.jl/issues/8
• Instructions on porting things from Nabla Port sensitivity definitions to ChainRules invenia/Nabla.jl#177
  • We need to write similar ones for Zygote and Tracker.
• https://invenia.github.io/Nabla.jl/latest/pages/autodiff.html

[oxinabox]

Probably not a good first issue

[nickrobinson251]

🤷 trying to write some documentation was how I got started with the package

[oxinabox]

Closed by #103