Split long pages into subsections

Copying from bluerobotics/BlueOS-docs#46:

> @dgudiel has pointed out that it can be cumbersome to navigate very long pages (like the Advanced Usage page), and that their existence somewhat under-utilises the high level navigation and understanding of available content provided by the left sidebar, while overloading the in-page navigation utility of the right sidebar.
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> While there is some value in avoiding excessive depth levels, "extra pages don't cost anything".