Improve 5 min intro documentation

[olivhoenen]

As imas_core will not be open source at the time of v2 release a couple of improvements in IMAS-Python shall be added, especially in the 5 min intro section:

• shall mention imas_core requirement in sections that depend on it
• shall demonstrate writing/reading IDS with netCDF (without imas_core)
• shall explain the diff in I/O with and without imas_core

[olivhoenen]

Available in 2.0.0 doc (https://imas-python.readthedocs.io/en/stable/intro.html#load-and-store-an-ids-to-disk-with-imas-core)