Make at-ref links more flexible

[mortenpi]

Allows at-ref links of the form

[the function `g`](@ref MyPackage.g)
[something-something](@ref "Section On Something")

which were previously not allowed. Previously you were only allowed to link either to an existing id (e.g. created by at-id, or by Documenter itself from a heading), or link to a docstring, but only if the whole label was code.

Fix #781, close #1767.