[Feature Request] Validate absolute paths to .md files

**Is your feature request related to a problem? Please describe.**
As documented at https://dotnet.github.io/docfx/docs/links-and-cross-references.html#link-to-local-files, absolute paths are not validated for broken links.

**Describe the solution you'd like**
When an absolute link references a markdown file, report if the link is broken. Nobody wants to expose .md files directly, right?

**Describe alternatives you've considered**
Telling everyone not to use absolute links to .md files. We found a large number of broken links in Google Search Console because developers were unaware of this pitfall.

**Additional context**
Example of relative link:
```
[other](../other/missing.md#some)
```
Example of absolute link:
```
[other](/other/missing.md#some)
```