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.

docs/book/book.toml

@@ -0,0 +1,34 @@
+[book]
+title = "heuropt — the user guide"
+description = "A practical Rust toolkit for heuristic single-, multi-, and many-objective optimization."
+authors = ["Stephen Waits"]
+language = "en"
+src = "src"
+
+[build]
+build-dir = "../../target/book"
+create-missing = false
+
+[output.html]
+default-theme = "rust"
+preferred-dark-theme = "navy"
+git-repository-url = "https://github.com/swaits/heuropt"
+edit-url-template = "https://github.com/swaits/heuropt/edit/main/docs/book/{path}"
+site-url = "/heuropt/"
+no-section-label = true
+
+[output.html.fold]
+enable = true
+level = 1
+
+[output.html.search]
+enable = true
+limit-results = 30
+teaser-word-count = 30
+use-boolean-and = true
+
+[output.html.print]
+enable = true
+
+[rust]
+edition = "2024"