[ntuple] Provide tutorial for (envisioned) framework usage

[hahnjo]

It would be good to provide a tutorial how we (RNTuple developers) envision the APIs to be used by experiment frameworks. Relevant classes and methods that come to mind:

• Creation of (bare) RNTupleModel and RFieldTokens (for parallel writing, see also [ntuple] Improve field token usage for parallel writing #16236)
• Creation of (multiple) RNTupleWriter / RNTupleParallelWriter when Appending to a single TFile
• For parallel writing, creation of RNTupleFillContext per thread or another granularity
• Creation of (potentially multiple) REntrys and using BindRawPtr() to fill objects
• Using FillNoCommit(), RNTupleFillStatus::ShouldCommitCluster(), FlushColumns() (see [ntuple] Method to prepare cluster commit / flush column write buffers #16241) and CommitCluster() to reduce the time in critical sections