feat(edint-fotovoltaica): add 5 candidate models for photovoltaic systems

[albertoabellagarcia]

Summary

Translates the EDINT Photovoltaic System Ontology (v0.2.1) from
https://github.com/EDINT-Ontologia/edint-ontologia-fotovoltaica — solar
installations on municipal infrastructure elements, and measurement of
electricity generation, consumption, accumulation and grid export.

Five entities (out of ~35 referenced classes)

The source file bundles a large number of imported/referenced external
classes for context — edint-infraestructura's own Facility/
InfrastructureElement, edint-aparcamiento's Parking (interestingly
under its new edint.es namespace — suggesting that ontology may have a
newer version too; out of scope here), SOSA/SAREF, QUDT, W3C Time, and
Spain's territorial hierarchy. None of these are modeled, consistent with
this candidate family's established practice. Only 5 classes are native:

• PhotovoltaicSystem: status, usage type (grid/stand-alone/hybrid),
  installed power, installation organization, in-service date.
• PhotovoltaicMeter: Sensor+Meter pattern, same treatment as
  CountingSensor/AccessSensor.
• PhotovoltaicGeneration: SOSA Observation pattern, same as
  CountingObservation/TrafficObservation. The class comment covers
  generation/consumption/storage/export, but the source declares no
  categorization property for which is which — expected to be conveyed via
  observedProperty's target concept instead.
• PhotovoltaicSystemContract: applies-to, contract holder, rate.
• PhotovoltaicSystemRate: thin entity, no source-declared properties.

Two things worth flagging

• Unlike its siblings, this ontology has already migrated to the
  edint.es namespace — no pending namespace-mismatch issue here.
• Unlike its siblings, this repository has no real example data at all
  (no examples/ or mappings/ directory) — all 5 examples are synthetic.

A documented gap, not acted on

The source declares hasPhotovoltaicSystem with domain
edint-infraestructura's InfrastructureElement — the forward relationship
from an infrastructure element to its photovoltaic system. Mirroring how
Facility gained 3 properties in its own v0.2.1 update, it should ideally
gain this one too. Not added here, to keep this PR scoped to the
fotovoltaica ontology itself rather than reopening edint-infraestructura
again — noted in standard-metadata.yaml as a follow-up opportunity.

Testing

• All schema.json/context.jsonld/example files parse as JSON, all 5
  example.json validate against their own schema.json.
• standard-metadata.yaml parses as YAML.
• Candidates index regenerated.

Note: based on current master, which doesn't yet include the still-open
#6/#7/#8 — no dependency between any of them.