Add insert example to docs

Nobody has claimed this yet.

Hello! From experience, inserting into SQLite databases has been a common hurdle for people new to Gleam (for the people that are using SQLite of course). What do you think of updating the docs somewhere to show an example of running an insert statement with a `sqlight.query` call?

Maybe something like this, moving one part of the data into a separate insert statement?

```gleam
import gleam/dynamic/decode
import sqlight

pub fn main() {
use conn <- sqlight.with_connection(":memory:")
let cat_decoder = {
use name <- decode.field(0, decode.string)
use age <- decode.field(1, decode.int)
decode.success(#(name, age))
}

let sql = "
create table cats (name text, age int);

insert into cats (name, age) values
('Nubi', 4),
('Biffy', 10);
"

let assert Ok(Nil) = sqlight.exec(sql, conn)

let sql = "insert into cats (name, age) values (?, ?)"

let assert Ok([]) =
sqlight.query(
sql,
on: conn,
with: [sqlight.text("Ginny"), sqlight.int(6)],
expecting: decode.int,
)

let sql = "
select name, age from cats
where age < ?
"

let assert Ok([#("Nubi", 4), #("Ginny", 6)]) =
sqlight.query(sql, on: conn, with: [sqlight.int(7)], expecting: cat_decoder)
}

```

Start by locating the project's documentation entry point and existing SQLite or sqlight.query examples. Add a runnable example showing an insert with parameter values, then verify the surrounding documentation remains consistent; done means newcomers can see how to execute an insert and retrieve the resulting row.

Written by the indexing model from the issue text.