main_event_loop.md: Port ashpd features from async-std to async-io

[ratijas]

async-std is no longer a feature of ashpd crate, and async-io replaced it. In fact, the whole async-std is deprecated in favor of smol.

[ratijas]

Looking into ashpd source code:

#[cfg(all(all(feature = "tokio", feature = "async-io"), not(doc)))]
compile_error!("You can't enable both async-io & tokio features at once");
#[cfg(all(not(feature = "tokio"), not(feature = "async-io"), not(doc)))]
compile_error!("Either the `async-io` or the `tokio` feature has to be enabled");

I don't know the whole story, but it looks like tokio XOR async-io. I'm not sure how smol fits into this picture; I guess smol is a direct alternative to gio event loop?

Also, account feature needs to be enabled for the presented example to work.

#[cfg_attr(docsrs, doc(cfg(feature = "account")))]
#[cfg(feature = "account")]
pub mod account;

[ratijas]

Also the example uses reqwest@0.12, but 0.13.2 is available, and 0.13 has brought breaking changes in features:

rustls is now the default TLS backend, instead of native-tls.
rustls-tls has been renamed to rustls.

[Hofer-Julian]

Can you please check this out @bilelmoussaoui?

[ratijas]

There are couple more recommendations of async-std on the page, they should probably be either dropped or replaced with async-io depending on context. I'm still figuring this out.