Expose Factsheets as plugins for Datasets

**Description**
Some datasets in dissemination portals do have a Factsheet that exposes numerical value to characterise aspects of the datasets. Here is an example below taken from the Hippocampus Hub:
https://www.hippocampushub.eu/model/experimental-data/neuronal-morphology/?instance=011127HP1&layer=SLM&mtype=SLM_PPA

Here are some examples of factsheets found in other dissemination portals:
- SSCx Morpho: https://bbpteam.epfl.ch/sscx-portal/data/exp-morphologies/factsheets/C060998B-P4/morphology_factsheeet.json
- SSCx Ephys: https://bbpteam.epfl.ch/sscx-portal/data/exp-trace-factsheets/C170101B-MT-C1/factsheet.json
- SSCx Neuron Model: https://bbpteam.epfl.ch/sscx-portal/data/memodel-factsheets/L4_TPC/cADpyr/S1FL/L4_TPC_cADpyr_1/me_type_factsheet.json
- Microcircuit: https://bbpteam.epfl.ch/sscx-portal/data/model-data/REGION/S1FL/Central/CircuitLayers/4/factsheet.json

It is unclear at this stage if the data format used in existing portals is standardised but that discussion could be included in this work in order to provide a plugin that can render this information in Fusion.

**Acceptance Criteria**
- Find out what KG entity can have Factsheets, only Dataset, others ?
- A data format is defined to support factsheets (coordinated with relevant BBP teams)
- A Nexus data model is agreed upon to enable the discovery of Factsheets associated to Datasets
- A plugin is developed to
- consume Factsheet and present them in Fusion
- numerical values are rounded to 3 decimal (TDB confirm if that's fixed or to be configured)
- check if tooltip support if needed during discussions (to explain the metric and potentially to show non rounded numerical value)
- download the corresponding factsheet data file
- Update plugin documentation with data model and format expectations

Start by reviewing the linked factsheet examples and the acceptance criteria, then confirm which KG entities and Nexus associations are in scope. Coordinate the factsheet format and data model with the relevant BBP teams before implementing the Fusion plugin. Done means factsheets can be rendered and downloaded, with the agreed numerical-value and tooltip behavior documented.

Written by the indexing model from the issue text.