* Move the extended documentation of code blocks to a separate page.
* Merge "code playgrounds" documentation to be a section of that page.
* Document copy widget on code blocks.
* This commit changes how we refer to "```python" type syntax for code
blocks. Instead of being called a syntax highlighting label, this is
now referred to as a "language tag", since it serves both syntax
highlighting and playgrounds.
* Remap all the links.
* Advertise this new page in various places that previously did not have a link.
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