Add the notion of documentation file exporters.

[Mpdreamz]

This allows us to have different implementations of how a DocumentationFile gets exported to the output folder.

Right now we have two implementations:

• Default
• Noop

The noop exporter can be used by passing --metadata-only to docs-builder.

This will only emit our metadata files to the output folder.

This is primarely build for https://github.com/elastic/asciidocalypse/pull/140 to ensure it can build and publish links but we no longer care about the html output.

[Copilot]

Pull Request Overview

This PR introduces the concept of documentation file exporters to allow different implementations for exporting documentation files. It provides a default exporter and a no-op exporter (triggered via the metadataOnly flag), updates the state tracking to record the exporter in use, and adjusts the CLI commands and documentation generation code accordingly.

• Introduces the DocumentationFileExporter interface and its default implementation.
• Implements a NoopDocumentationFileExporter for metadata-only builds.
• Updates GenerationState, CLI commands, and DocumentationGenerator to utilize the exporter.

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/Elastic.Markdown/Exporters/DocumentationFileExporter.cs	Adds the exporter interface and base implementation for processing and copying documentation files.
src/Elastic.Markdown/Exporters/NoopDocumentationFileExporter.cs	Implements a no-op exporter for metadata-only exports.
src/Elastic.Markdown/IO/State/GenerationState.cs	Records the exporter name in the generation state.
src/docs-builder/Cli/Commands.cs	Adds a new CLI parameter 'metadataOnly' to enable the noop exporter.
src/Elastic.Markdown/DocumentationGenerator.cs	Refactors file and resource processing to use the exporter abstraction.

Comments suppressed due to low confidence (1)

src/Elastic.Markdown/Exporters/DocumentationFileExporter.cs:16

• [nitpick] Consider renaming the parameter 'token' to 'cancellationToken' for clarity and consistency with .NET conventions.

Task ProcessFile(DocumentationFile file, IFileInfo outputFile, Cancel token);