Add a way to put runnable RPL code in the documentation

[c3d]

Suggested by @Wiljea: Being able to put anchor containing RPL code in the help file, that would execute when you select them, would make it much easier to share examples of code.

An alternative would be to automatically be able to select code in between markdown code blocks. The following would show as code, and then if ENTER is pressed while the cursor is there, the code would be put in the editor.

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