rework placeholders

Placeholders have been modified.

You can now add them from any place with @include placeholder() accepts any number of arrays as string or as a map, if string uses the same name both for the placeholder as for the call to the function.

Now it does not allow creating placeholders once we have added the include marscss.

@include help

Now it gives detailed information about the placeholders that we have assets.

Big changes in @include marscss

Now only generates the placeholders, the arguments have been eliminated and it does not allow to be called more than once.

The documentation has been structured a bit but there is a lot of work here