Restructure complex Rust guides into new section(s)

[vados-cosmonic]

The Rust guides that existed as part of the somewhat overly-long intro section (and were removed as part of #310 ) need to be re-added into a section that better matches the content.

The PR sheds more light on changes that need to be made, but in general:

• Create new sections (that will have language subsections) for more involved guides (ex. creating your own custom host, resources, etc)
• Create Rust as a section there
• Enable use of fold if possible to avoid taking up too much left-hand-side space

Going forward, this will allow us to keep the initial tutorial very short (building an adder) and then from there people can move on to slightly more complex use cases.