docs: fix nesting of next sections under examples (#7159)

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.

Author: edev

tokio/src/lib.rs

@@ -302,7 +302,7 @@
 //! }
 //! ```
 //!
-//! ## Feature flags
+//! # Feature flags
 //!
 //! Tokio uses a set of [feature flags] to reduce the amount of compiled code. It
 //! is possible to just enable certain features over others. By default, Tokio
@@ -340,7 +340,7 @@
 //! _Note: `AsyncRead` and `AsyncWrite` traits do not require any features and are
 //! always available._
 //!
-//! ### Unstable features
+//! ## Unstable features
 //!
 //! Some feature flags are only available when specifying the `tokio_unstable` flag:
 //!
@@ -392,7 +392,7 @@
 //! [unstable features]: https://internals.rust-lang.org/t/feature-request-unstable-opt-in-non-transitive-crate-features/16193#why-not-a-crate-feature-2
 //! [feature flags]: https://doc.rust-lang.org/cargo/reference/manifest.html#the-features-section
 //!
-//! ## Supported platforms
+//! # Supported platforms
 //!
 //! Tokio currently guarantees support for the following platforms:
 //!
@@ -418,7 +418,7 @@
 //!
 //! [mio-supported]: https://crates.io/crates/mio#platforms
 //!
-//! ### `WASM` support
+//! ## `WASM` support
 //!
 //! Tokio has some limited support for the `WASM` platform. Without the
 //! `tokio_unstable` flag, the following features are supported:
@@ -440,7 +440,7 @@
 //! immediately instead of blocking forever. On platforms that don't support
 //! time, this means that the runtime can never be idle in any way.
 //!
-//! ### Unstable `WASM` support
+//! ## Unstable `WASM` support
 //!
 //! Tokio also has unstable support for some additional `WASM` features. This
 //! requires the use of the `tokio_unstable` flag.