Add `Markdoc`, `.mdoc` about linguist

[Alhadis]

Most popular extensions: .mdoc, .md

Ugh, this project picked two file extensions already in-use by existing markup languages (Roff and Markdown). Assuming that Markdoc achieves sufficient usage on GitHub, adding support for it is going to be a pain-in-the-arse… 😞

[jasikpark]

This is why every language should specify itself at the top of the doc like XHTML xD

[spenserblack]

If I understand correctly, Markdoc boils down to Markdown plus an extended syntax like {% this %}? If so I imagine a lot of repositories that use Markdown with other template tools that use this Jinja-like syntax can get misidentified as Markdoc very easily.

I noticed that HTML+Django (alias of Jinja) does not have the .html extension, even though it's commonly used (see nearly every Django project). I assume this is intentional since the syntax would be too similar to tell them apart. Perhaps Markdoc should be treated similarly, and not have the .md extension registered for it?