feat: v0.5.0 — comprehensive documentation release

Theme: documentation and project polish. No public-API changes; this
is the v0.5 release that elevates heuropt's docs/onboarding/governance
to bar-setting status.

Adds:
- mdbook user guide at docs/book/ with intro, getting-started,
  defining-problems, choosing-an-algorithm, cookbook (7 recipes),
  comparison vs other libraries, stability/SemVer, migration guides.
  Deploys to https://swaits.github.io/heuropt/ via .github/workflows/
  docs.yml.
- Runnable rustdoc examples on every algorithm (35 of them), all
  exercised by cargo test --doc.
- Three real-world examples: portfolio.rs (multi-obj with budget
  constraint), hyperparam_tuning.rs (BO + TPE), scheduling.rs
  (permutation via SA + SwapMutation against Smith's-rule oracle).
- Governance: CONTRIBUTING.md, SECURITY.md, CODE_OF_CONDUCT.md
  (adopting builderscode.org's Builder's Code of Conduct), GitHub
  issue templates, PR template.

Polishes:
- README hero with badges + user-guide link.
- lib.rs crate-level docs.
- CHANGELOG entry for 0.5.0.

Bumps Cargo.toml to 0.5.0.

src/algorithms/hyperband.rs

@@ -52,6 +52,38 @@ impl Default for HyperbandConfig {
 /// low budget), later brackets favor exploitation (fewer configs run
 /// near the max budget). The single best result across all brackets
 /// is returned.
+///
+/// # Example
+///
+/// ```
+/// use heuropt::prelude::*;
+/// use heuropt::core::partial_problem::PartialProblem;
+///
+/// struct Tuning;
+/// impl PartialProblem for Tuning {
+///     type Decision = Vec<f64>;
+///     fn objectives(&self) -> ObjectiveSpace {
+///         ObjectiveSpace::new(vec![Objective::minimize("loss")])
+///     }
+///     fn evaluate_at_budget(&self, x: &Vec<f64>, budget: f64) -> Evaluation {
+///         // Pretend a model where more budget = lower loss.
+///         let loss = x[0].powi(2) + x[1].powi(2) + 1.0 / (budget + 1.0);
+///         Evaluation::new(vec![loss])
+///     }
+/// }
+///
+/// let mut opt = Hyperband::new(
+///     HyperbandConfig {
+///         max_budget: 27.0,
+///         eta: 3.0,
+///         max_brackets: 4,
+///         seed: 42,
+///     },
+///     RealBounds::new(vec![(-1.0, 1.0); 2]),
+/// );
+/// let r = opt.run(&Tuning);
+/// assert!(r.best.is_some());
+/// ```
 pub struct Hyperband<I, D>
 where
     D: Clone,