Split long pages into subsections

[ES-Alexander]

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.

While there is some value in avoiding excessive depth levels, "extra pages don't cost anything".