Allow reusable content to be used in any context

[rogerluan]

Currently reusable content blocks are basically just reusable paragraphs that can't be properly embedded in more complex structures. Some examples:

• I can't add an inline reusable content
• I can't add a reusable content inside a code block
• I can't add a reusable content inside a hint box
• I can't nest a reusable content inside another reusable content

I tried doing these things both via the web UI but also programmatically, and they're all broken in one way or another.

Some use cases:

• Having a reusable content that is modified programmatically (e.g. SDK Version) and mentioned inline within paragraphs, code blocks, etc.
• Having sentences that only make sense to be mentioned inside larger paragraphs, and not as standalone paragraphs on their own.
• Giving greater emphasis to a reusable content by putting it inside a hint box in one place, but not in another.
• Highly reusable content with multiple levels where nesting makes sense.

Happy to give any more context here.