Update README.md documentation

[maloleroy]

The README.md contained outdated or broken links, an outdated capture of the IBM roadmap to quantum-centric supercomputers, and an outdated code snippet. Notably:

• IBM Quantum Experience is now named IBM Quantum Platform (source)
• The landing page for IBM Quantum Platform changed
• The link to IBM roadmap to quantum-centric supercomputers, although containing 2025 in the URL, was only up-to-date until 2024. It referred to another page for the 2025 version
• OpenQASM's repository changed from the Qiskit organization to the openqasm organization (https://github.com/Qiskit/openqasm -> https://github.com/openqasm/openqasm)
• When OpenQASM-2 was specifically referenced, link to the OpenQASM2.x branch in OpenQASM's repo (the main branch refers to 3.x). On other cases, the OpenQASM repository main page was used
• The Qiskit code snippet to generate QASM code from a QuantumCircuit was updated (assuming OpenQASM-2 use, not OpenQASM-3)