docs: make Enzyme warning more visible

[gdalle]

• Turn the Enzyme warning on the README into a GitHub admonition for more visibility:
• Translate it to a Documenter.jl admonition when building the docs.
• Add a related warning to the backends page

The warning now looks like this on the README:

Caution

Note that in some cases, going through DifferentiationInterface.jl might be slower or cause more errors than a direct call to the backend’s API. This is especially true for Enzyme.jl, whose handling of activities and multiple arguments is not fully supported here. We are working on this challenge, and welcome any suggestions or contributions. Meanwhile, if differentiation fails or takes too long, consider using Enzyme.jl through its native API instead.

Related:

• Add DI warning to AutoEnzyme SciML/ADTypes.jl#109
• Add warnings EnzymeAD/Enzyme.jl#2415

[gdalle]

cc @wsmoses, trying to make this as visible and explicit as I can

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.93%. Comparing base (f07b6ec) to head (f53bc1a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #808   +/-   ##
=======================================
  Coverage   97.93%   97.93%           
=======================================
  Files         128      128           
  Lines        7693     7693           
=======================================
  Hits         7534     7534           
  Misses        159      159           

Flag	Coverage Δ
DI	98.81% <ø> (ø)
DIT	95.61% <ø> (ø)

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