docs: add browser, node, and nextjs examples

[darioAnongba]

Description

Closes #23.

This PR adds ready-to-copy onboarding examples for the published JavaScript packages:

• examples/browser-vite for a minimal browser wallet setup with Vite
• examples/node-wallet for descriptor-based wallet creation, Esplora sync, PSBT signing, and transaction broadcast in Node.js
• examples/nextjs for the client-only loading pattern inside a Next.js app
• examples/README.md as the landing page for safety notes and example selection
• README links to the new examples section
• a mandatory CHANGELOG.md entry under [Unreleased]

It also hardens CI by retrying the wasm-pack download step in the GitHub workflows after repeated HTTP 504 failures from GitHub's release endpoint.

Notes to the reviewers

• The browser-facing examples intentionally use throwaway signet descriptors copied from the repository test fixtures so the snippets stay self-contained.
• The docs explicitly warn against shipping private descriptors or xprvs in production browser bundles.
• Local validation run before opening the PR: cargo fmt --check, cargo clippy --all-features --all-targets -- -D warnings, cargo test --all-features, and corepack yarn lint.
• The CI retry change was added only after the first PR runs failed on the same external wasm-pack download step.

Changelog notice

Added an examples/ directory with browser (Vite), Node.js, and Next.js tutorials, plus README links for quicker onboarding.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR