Allow YAML front matter in the middle of the document

[yufeih]

This PR enables customizing the YAML front matter extension to allow it to appear in the middle of the markdown file.

The docfx overwrite file feature leverage this feature to embed multiple YAML fragments in the same markdown file. With this change, docfx can be fully migrated to markdig.

Fixes: dotnet/docfx#8441

[coveralls]

Pull Request Test Coverage Report for Build 4252961669

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 11 of 11 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 93.295%

---

Totals
Change from base Build 4191078997:	0.002%
Covered Lines:	25932
Relevant Lines:	27187

---

💛 - Coveralls

[yufeih]

Thank you for merging @xoofx !