docs: improve project discoverability and contributor onboarding

[vitormattos]

Summary

Improve how PDF Elements presents itself to potential users and contributors while keeping the README focused on project discovery.

Changes

• Rework the README around the project's purpose, value proposition and real use cases.
• Keep the README as a project overview and navigation page rather than a tutorial.
• Move installation and first-use guidance to docs/GETTING_STARTED.md.
• Move the detailed API and theming reference to docs/API.md.
• Keep contributor workflow and development checks in CONTRIBUTING.md.
• Add npm and CI badges.
• Clarify the boundary between the interaction layer and cryptographic PDF signing.
• Fix contributor setup instructions, including the invalid npm run build:lib reference.
• Improve package metadata, homepage and npm keywords for discoverability.
• Add structured bug and feature request forms.
• Add an issue chooser with links to the demo and LibreSign.
• Add a pull request template.

The README deliberately does not duplicate license information already represented by the repository license files and GitHub metadata.