Release v0.3.1: docs-only tutorial enhancement

[mmogib]

Release v0.3.1 — docs-only tutorial enhancement

Summary

Adds a new Tutorial page (docs/src/tutorial.md) to the manual,
slotted between Quickstart and Algorithm. Live-evaluated via Documenter
@example blocks so the rendered HTML shows real numbers, an inline
convergence plot, and a results DataFrame.

No source-code changes; no API additions or removals; 242/242 tests
pass unchanged. Drop-in upgrade from v0.3.0.

What's new

The tutorial walks an end-to-end usage flow with emphasis on the
callback architecture (which v0.3.0 introduced but didn't have a
focused page for):

• § 1 Setup — minimum-viable solve(NonlinearProblem(F, x0), DFProjection())
• § 2 Choosing components — overrides for linesearch, inertial, etc.
• § 3 Built-in observer callbacks — LoggingCallback (per-iter
  table) + HistoryCallback (collect data, plot inline convergence
  curve via Plots.jl)
• § 4 Stopping criteria as callbacks — AnyOf(AbsResidualTol, MaxIters, MaxTime) composition; documents the
  AbstractStoppingCriterion <: AbstractCallback design
• § 5 Writing a custom callback — IterateSnapshotCallback from
  scratch, showing the on_event!(cb, cache, event::Symbol) contract
  end-to-end (subtyping, event handling, mutating-cache.x discipline)
• § 6 Comparative sweep — small 3-problem × 3-line-search loop,
  results aggregated into a DataFrame rendered inline

Companion changes

• docs/Project.toml: added Plots, DataFrames, LinearAlgebra
  — needed by the new @example blocks (rendered convergence plot in
  § 3, results table in § 6).
• docs/make.jl: added "Tutorial" => "tutorial.md" to the pages
  list between Quickstart and Algorithm.

Test plan

• Local doc build (julia --project=docs docs/make.jl) — all
  @example tut blocks evaluate cleanly; convergence plot renders
  via GR; DataFrame renders inline.
• Library test suite unchanged: 242/242 pass.
• CI: GitHub Actions (CI.yml) — same matrix as v0.3.0; should be
  identical.

Notes for AutoMerge

• PATCH bump (0.3.0 → 0.3.1) — not a BREAKING release per Julia semver.
  release.sh correctly classified this and did not require
  --breaking/--notes-file.
• No new direct deps in the library's Project.toml. Docs-only deps
  (Plots, DataFrames, LinearAlgebra) are confined to docs/Project.toml
  and do not affect AutoMerge's compatibility checks.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.74%. Comparing base (094c18c) to head (fb15d52).
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Additional details and impacted files

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  Coverage   90.74%   90.74%           
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  Files          12       12           
  Lines         670      670           
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  Hits          608      608           
  Misses         62       62           

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