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GH-40841: [Docs][C++][Python] Add initial documentation for RecordBatch::Tensor conversion (#40842)

### Rationale for this change

The work on the conversion from `Table`/`RecordBatch` to `Tensor` is progressing and we have to make sure to add information to the documentation. 

### What changes are included in this PR?

I propose to add

- new page (`converting_recordbatch_to_tensor.rst`) in the `cpp/examples` section,
- added section (Conversion of RecordBatch do Tensor) in the `docs/source/python/data.rst`

the content above would be updated as the features are added in the future (row-major conversion, `Table::ToTensor`, DLPack support for `Tensor` class, etc.)

### Are these changes tested?

It will be tested with the crossbow preview-docs job.

### Are there any user-facing changes?

No, just documentation.
* GitHub Issue: #40841

Lead-authored-by: AlenkaF <frim.alenka@gmail.com>
Co-authored-by: Alenka Frim <AlenkaF@users.noreply.github.com>
Co-authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>