Add dev guide to docs

[gdalle]

Preview: https://gdalle.github.io/DifferentiationInterface.jl/DifferentiationInterface/previews/PR317/

DI docs

• Add a full dev guide for contributors who want to add new operators or new backends
• Clarify which operators support two-argument functions

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.04%. Comparing base (9fb32cd) to head (2806568).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #317   +/-   ##
=======================================
  Coverage   96.04%   96.04%           
=======================================
  Files          95       95           
  Lines        4403     4403           
=======================================
  Hits         4229     4229           
  Misses        174      174           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.