docs: replace the CC BY 4.0 claim with the TABULA usage rules

[jravani]

The repository said the TABULA workbooks are licensed under CC BY 4.0. They are not. The provider's Rules for Usage of the TABULA "Building Typology" Approach (Systematics, Data and Tools) by Third Parties read:

The usage of the TABULA approach, data, and tools in research projects, theses, and software applications by third parties is intended and desired. Only non-exclusive utilisations are possible.

A condition for usages of any kind (files, datasets, pictures, ...) is that "IEE Projects TABULA + EPISCOPE (www.episcope.eu)" is visibly mentioned as the source.

Changes

File	Change
ATTRIBUTIONS.md	License: CC BY 4.0 replaced by the credit line and the terms of use; the derivative workbook is described as under the same rules
data/README.md	the licence row and both CC BY sentences replaced; states the files are not under the repository's MIT licence
README.md	licence section states MIT covers the code, not the TABULA data; acknowledgement carries the exact credit line
docs/index.md	credit line added beside the TABULA link
docs/open-source-checklist.md	checklist item names the TABULA usage rules instead of CC BY 4.0

Both workbooks stay in data/ and in the ignis-build-db image. The rules allow that, with the credit.

The README licence line had an em dash, replaced by a full stop since the line was edited anyway.

Checks

mkdocs build --strict passes. No remaining CC BY 4.0 reference in the repository other than the generic mention in the checklist's guidance text and the Contributor Covenant licence in CODE_OF_CONDUCT.md.