Change ToRouteSegments to borrow self

[ealmloff]

This changes the ToRouteSegments trait to take &self instead of self and adds examples. The router macro currently expects the borrowed version of the type used in the macro to implement ToRouteSegments which can cause confusing errors if you implement it for the type itself.

Currently, you need to implement the trait like this:

#[derive(Clone, Routable, Debug, PartialEq)]
pub(crate) enum Route {
    #[route("/:..search")]
    Bookmarks { search: Segments },
}

#[component]
fn Bookmarks(search: Segments) -> Element {
    todo!()
}

#[derive(Clone, Debug, PartialEq, Eq)]
struct Segments;

impl FromRouteSegments for Segments {
    fn from_route_segments(_: &[&str]) -> Result<Self, Self::Err> {
        todo!()
    }

    type Err = String;
}

impl ToRouteSegments for &Segments {
    fn display_route_segments(self, _: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        todo!()
    }
}

After this PR, you can implement the trait like this:

#[derive(Clone, Routable, Debug, PartialEq)]
pub(crate) enum Route {
    #[route("/:..search")]
    Bookmarks { search: Segments },
}

#[component]
fn Bookmarks(search: Segments) -> Element {
    todo!()
}

#[derive(Clone, Debug, PartialEq, Eq)]
struct Segments;

impl FromRouteSegments for Segments {
    fn from_route_segments(_: &[&str]) -> Result<Self, Self::Err> {
        todo!()
    }

    type Err = String;
}

impl ToRouteSegments for Segments {
    fn display_route_segments(&self, _: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        todo!()
    }
}