Add docs for nested router

[Madoshakalaka]

This is about:

• A typo
• Innaccurate/misleading documentation (e.g. technically incorrect advice)
• Undocumented code
• Outdated documentation
• Other

Problem
Current documentation doesn't make it obvious how routers can nest within each other.

here is an example of how this could be done by @mc1098
#2097 (comment)

However, it needs to be updated because of recent updates to the router.

[Madoshakalaka]

I'm interested in doing this

[Madoshakalaka]

It's not actually "undocumented code". Because nested router is not explicitly a feature in the code but just a way how routers can be used together. It's just unclear how this should be done and some instruction is needed.

[Madoshakalaka]

May I also add a nested_router example alongside the current router example?

[ranile]

Related to #1853. It might be worth tackling that issue first as that improves the ergonomics of nesting routers. Right now, the experience of using nested routers isn't ideal.