Allow extensions to inject IDocumentationFileExporter and control the relative path published in links.json

[Mpdreamz]

Fixes: elastic/detection-rules#4508

Ensures the detection rule extension emits to .artifacts/docs/html not .artifacts/docs because the rule references come from ../rules.

This also ensures links.json advertises the virtual markdown files not the toml files.

See bad links.json file here: https://docs-v3-preview.elastic.dev/elastic/detection-rules/pull/4508/links.json

Excerpt from local build:

"rules/cross-platform/initial_access_zoom_meeting_with_no_passcode.md": {
      "anchors": [
        "setup",
        "investigation-guide",
        "triage-and-analysis",
        "investigating-zoom-meeting-with-no-passcode",
        "possible-investigation-steps",
        "false-positive-analysis",
        "response-and-remediation",
        "rule-query"
      ]
    }