Update gradient interface, support AbstractDifferentiation

[lostella]

1. Remove dependency on Zygote in favor of AbstractDifferentiation (#71, #80, cc @gdalle)
2. Update the interface to gradient computation used by algorithms, see value_and_pullback (similar to AbstractDifferentiation value_and_pullback_function)
3. Update stepsize backtracking routines, to only evaluate gradients at the end (made possible by the changes in 2.) speeding things up (need to benchmark this but it’s reasonable to expect some speedup)
4. Update tests and documentation
5. Add justfile (see just)

Work in progress, opening for feedback and to take a look at CI workflows.

TODOs

• [x] expand on AD in README and docs landing page
• [x] update benchmarks code
• [ ] benchmark changes to backtracking

[codecov[bot]]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (4d07b28) 89.84% compared to head (fd98409) 89.03%.

:exclamation: Current head fd98409 differs from pull request most recent head 68a80a0. Consider uploading reports for the commit 68a80a0 to get more accurate results

Files	Patch %	Lines
src/algorithms/li_lin.jl	66.66%	2 Missing :warning:
src/algorithms/panoc.jl	75.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #90 +/- ## ========================================== - Coverage 89.84% 89.03% -0.81% ========================================== Files 21 20 -1 Lines 955 985 +30 ========================================== + Hits 858 877 +19 - Misses 97 108 +11 ```

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