[css-contain] property names linkified

[ewilligers]

'counter-increment' and 'content' property names are now working links.

resolves #2800

[frivoal]

Sorry, this needs to be reverted:

1. linking to css-content instead of css2 is not appropriate. css-content is a known-to-be buggy draft. It is not as bad as it used to be, as some maintenance work has been done to it, but I don't think it has reached the point where it is more accurate than css2. Case in point: it claims that the content property applies to all elements, which is absolutely not true in existing implementations. css2 correctly says that it applies to ":before and :after pseudo-elements "

2. independently of the point above, neither css-content nor css-list-3 are mature enough to be accepted without strong justifications as normative references for a document which is trying to exit CR, so we will have to change these links anyway before next publication of css-contain

[frivoal]

sub-point on css-list-3: for counter-set, css-list-3 is vaguely acceptable, as:

1. counter-set is not defined anywhere else
2. it can be argued that counter-set is almost identical to counter-reset, which is in a REC.

That makes it possible to argue in favor of using css-list-3 as a reference for counter-set, but harder to argue in favor of it for things that exist in CSS2.

[frivoal]

Commit 26d5bf6 fixes this. And since it uses css2 rather than css21, we do get cross-spec links.