Bump Markdig from 0.39.1 to 0.40.0 in /src/Elastic.Markdown.Refactor (#925)

Bumps [Markdig](https://github.com/xoofx/markdig) from 0.39.1 to 0.40.0.
- [Release notes](https://github.com/xoofx/markdig/releases)
- [Changelog](https://github.com/xoofx/markdig/blob/master/changelog.md)
- [Commits](xoofx/markdig@0.39.1...0.40.0)

---
updated-dependencies:
- dependency-name: Markdig
  dependency-version: 0.40.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

src/Elastic.Markdown/Elastic.Markdown.csproj

@@ -54,7 +54,7 @@
     <PackageReference Include="DotNet.Glob" Version="3.1.3" />
     <PackageReference Include="Samboy063.Tomlet" Version="6.0.0" />
     <PackageReference Include="SoftCircuits.IniFileParser" Version="2.7.0" />
-    <PackageReference Include="Markdig" Version="0.39.1" />
+    <PackageReference Include="Markdig" Version="0.40.0" />
     <PackageReference Include="Microsoft.Extensions.Logging" Version="9.0.0" />
     <PackageReference Include="RazorSlices" Version="0.8.1" />
     <PackageReference Include="Slugify.Core" Version="4.0.1" />