Update defining blocks page and playground samples

[kimprice]

• Replaced all '%' parameter annotations with '$' (closes Update Playground samples and defining blocks documentation to use '$' instead of '%' for parameters #9211)
• Removed 'eg:' from JSDoc comments in favor of setting the default with PARAM.defl. The eg notation automatically sets the first value listed as the default value for the parameter. This indirect way of setting the default may cause confusion, so we are no longer advertising it.
• Added a sample for fixed instances of a class
• Added a sample for default array values
• Updated the classes sample to include an example with blockCombine
• Removed all the information on Blockly mutators. The mutator blocks do not fit with the look and feel of our other blocks, so we no longer want to advertise it.

Also closes #9175 because the corresponding section was removed as unnecessary.

[abchatra]

Nice set up changes.

[riknoll]

should probably get rid of the = type bit since param.shadow is preferred

[riknoll]

you might need to put quotes around AlarmSound.Annoy here... I'm not actually sure. The . might confuse our parser

[kimprice]

I checked it in the playground and it works fine.