docs: fix nesting of next sections under examples

[edev]

Promote the "Feature flags" and "Supported platforms" sections out from under "Examples", as they are not examples. Adjust their subsections accordingly. Expose these subsections via navigation sidebar.

Motivation

I believe the documentation subheadings under "Examples" might have been nested by mistake. It seems like they were meant to follow "Examples", and I suspect that no one noticed this mistake because the default page styling makes the nesting structure hard to see.

This matters for at least two reasons:

1. Accessibility, e.g. to users of screen readers.
2. The "Sections" navigation menu in the sidebar only renders # and ## headings. The headings "Unstable features", "WASM support", and "Unstable WASM support" are absent from this menu, and I believe they were meant to be present.

Solution

Promote each subsection under Examples by one level.

[maminrayej]

Using heading level 2 for "Feature flags" and "Supported platforms" is consistent with "Working With Tasks", "CPU-bound tasks and blocking code", and "Asynchronous IO". I guess "Examples" is using heading level 1 since it's an overall example of using tokio.

[Darksonn]

I guess those are part of the "tour of Tokio"

This layout makes sense to me, thanks.