Markdown AST linting

[mortenpi]

There are some common errors people make with Markdown that are pretty clearly errors, but parse into valid AST (everything does), and so go unnoticed (e.g. forgetting to have closing fences for code blocks). We could implement a linting mode for Documenter, as part of docchecks, that tries to catch these.

Two steps for a useful feature:

• A new linting step as part of docchecking. Key here is that it should be possible to disable this on its own (e.g. if you get false positives).
• The code fence closing linter should be relatively straightforward to implement.

Suggested by @giordano: JuliaLang/julia#48893 (comment)