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.

.github/ISSUE_TEMPLATE/docs.md

@@ -0,0 +1,24 @@
+---
+name: Docs issue
+about: Something in the README, mdbook guide, or rustdoc is wrong, missing, or unclear
+title: "docs: <one-line summary>"
+labels: documentation
+---
+
+## Where
+
+- [ ] `README.md`
+- [ ] mdbook user guide (chapter / section: ____ )
+- [ ] rustdoc on a specific item (path: ____ )
+- [ ] Examples (`examples/____.rs`)
+- [ ] CHANGELOG / migration guide
+- [ ] Other: ____
+
+## What's wrong
+
+<Concrete description: typo, broken link, outdated code sample,
+missing topic, unclear explanation, etc.>
+
+## What it should say (if you know)
+
+<Optional: proposed wording or correct content. Even a sketch helps.>