Revamp documentation

[gdalle]

Preview links:

• DI: https://gdalle.github.io/DifferentiationInterface.jl/DifferentiationInterface/previews/PR276/
• DIT: https://gdalle.github.io/DifferentiationInterface.jl/DifferentiationInterfaceTest/previews/PR276/

DI compat

• Bump version to 0.4.2

DI source

• Add more details to operator docstrings

DI docs

• Split the tutorial into "Basics" and "Sparsity"
• Add more details to the overview, renamed as "Operators"
• Add a page on "Preparation" detailing the choices in each backend
• Restructure the "API" page to have fewer sections

DIT docs

• Adjust the link to the DI docs
• Replace @repl with @example in the tutorial

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.81%. Comparing base (dc317b0) to head (3fd9caa).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #276   +/-   ##
=======================================
  Coverage   96.81%   96.81%           
=======================================
  Files          72       72           
  Lines        3740     3740           
=======================================
  Hits         3621     3621           
  Misses        119      119           

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