Improve constraints article

[ntrel]

Depends on #3486, please merge that first.

Add example of type specializations.
Improve wording.
Add subheadings.
Make some examples compilable.
Add links to IsExpression, CTFE, traits.
Note that a parameter specialization e.g. (T : float) accepts types that implicitly convert to float, and that using a constraint is necessary to prevent accepting an implicitly converting type.
Use const variable templates instead of template.
Make isAddable example runnable & use opBinary.
Explain __traits(compiles) and why using a function literal is useful vs T.init.
Move multiple parameter constraint example up.
Introduce isStack better.
Mention using static if to combine overloads.

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