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/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,32 @@
+<!--
+Thanks for the contribution! Please skim CONTRIBUTING.md if you
+haven't yet — it has the local-test checklist and the conventional-
+commits requirement.
+-->
+
+## What
+
+<One- or two-sentence summary. Focus on the *what* and *why*, not
+the *how*.>
+
+## Why
+
+<Motivation. Link the issue this resolves with `Closes #N` if
+applicable.>
+
+## Checklist
+
+- [ ] `cargo fmt --all`
+- [ ] `cargo clippy --all-targets --all-features -- -D warnings`
+- [ ] `cargo test` and `cargo test --all-features`
+- [ ] `cargo doc --no-deps --all-features` (with `-D warnings`)
+- [ ] Conventional-commit subject(s) (`<type>(<scope>): <summary>`)
+- [ ] If touching algorithm output: confirmed bit-identical results
+      via `cargo run --release --example compare`
+- [ ] If perf change: included gungraun before/after numbers in the
+      commit message
+- [ ] Updated CHANGELOG.md under `[Unreleased]` if user-visible
+
+## Anything else
+
+<Caveats, follow-ups, screenshots, perf numbers, etc.>