feat: add documentation url to diagnostics

[dweber019]

Motivation and Context

Add the documentation URL to the diagnostics interface.
See stoplightio/spectral#1488

Description

• added documentationUrl as optional property to interface IDiagnostic

Screenshot(s)/recordings(s)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• This PR's code follows as closely as possible the coding style/guidelines of this project.
• I have added error reporting and followed the error reporting guidelines.
• I have added event tracking and followed the event tracking guidelines.
• I have updated any relevant documentation accordingly to reflect this PR's changes.
• I have added automated tests (unit/integration/e2e/other) to cover my changes.
• All new and existing tests pass locally (excluding flaky CI tests).

[dweber019]

Or should this be solved in https://github.com/stoplightio/spectral/blob/develop/packages/core/src/types/spectral.ts#L13?