Improve new user documentation

[vilgotf]

Development best practices for twilight are currently hard to discover. For example, most bots developed with twilight will have a Context struct like this:

pub struct Context {
    pub cache: InMemoryCache,
    pub cluster: Cluster,
    pub http: Client,
    // etc.
}

Structuring one's code like this brings two major advantages:

1. Methods on Context need only take &self instead of InMemoryCache, Cluster etc.
2. You only need one Arc for multi-threading

But only /examples/lavalink-basic-bot implement this scheme, so it's not easily discoverable for new users.

lavalink-basic-bot is also the only example of a fully functional bot, I believe new users would find additional, longer examples useful.

• The book is also unmentioned in the main readme (Better discoverability of the book #2195).

[laralove143]

twilight's support for the tracing crate goes unnoticed unless people look at examples, most libs mention it, optionally linking to the readme or docs of the crate. i think this used to be the case with twilight, it'd be nice to have it back