Throw error if invalid link syntax is detected

[chrispcampbell]

Sometimes during the translation process (in Weblate) a space will be inserted into Markdown link syntax that will cause it to be parsed as plain text instead of a proper link.

For example:

This is a normal [link] (https://github.com).

This is a ref-style [link] [ref].

Currently we can only detect this manually (by inspecting the translation files or the resulting rendered pages). It would be better if we had docs-builder check for this case and throw an error if invalid link syntax is detected.