Add Label element to Related for keyword-based filtering

[Copilot]

FMU creators had no structured way to attach keywords to related files, making it impossible to filter references by use case (e.g., variant, SIL/HIL, target OS) without opening each file.

Changes

• XSD schema (fmi3LayeredStandardReferenceManifest.xsd): Added Label as a child element of Related (0..unbounded). New TLabel complex type with mandatory name and optional description attributes.
• Documentation (docs/index.adoc): Added "Related Child Elements" table and a "Labels" subsection describing the element and its attributes.
• Example (docs/examples/fmi_ls_ref_manifest_example.xml): Illustrates label usage for variant selection and simulation context filtering.

Example

<Related type="application/x-ssp-parameter-set" source="highend-params.ssv"
         role="parameter" description="Highend Model Parameters">
    <Label name="variant:highend" description="Highend variant parameters"/>
</Related>

<Related type="application/x-ma-ls-experiments" source="smoke-tests.exp"
         role="experiment" description="Smoke Tests of FMU">
    <Label name="simulation-type:SIL" description="Applicable for Software-in-the-Loop simulation"/>
    <Label name="os:Win64" description="Applicable for Windows 64-bit target"/>
</Related>