Improving and updating information about Qiskit Code Assistant

[cbjuan]

This PR introduces the following edit/updates:

1. Remove explicit references to the Qiskit version used for training the Assistant
2. Editing a text in the VSCode page that said that the Flex and On-prem plans have access to the assistant (only the premium plan has access to it), This wrong statement was only present in that page and not in the others
3. Introducing the availability of the VSCode extension with VSCode compatible editors (Codium, Cursor, etc.)
4. Introducing Qiskit HumanEval hard, links to QHE and QHE hard and new references

Fixes #4376

[qiskit-bot]

One or more of the following people are relevant to this code:

• @abbycross
• @beckykd
• cbjuan