rustdoc: remove `.borrow()` calls in Askama templates for `ItemTemplate` implementors

[nicklimmm]

Context:

• rustdoc: Add ItemTemplate trait and related functions to avoid repetitively wrapping existing functions #111946 (see section: existing discussion on how to solve this issue)
• Removing only .borrow() calls will raise compilation errors
• Initial idea of creating derive macros for ItemTemplate

Would like to hear more ideas to tackle this problem, but this is my proposal (rough idea):

• Blanket impl of ItemTemplate for references: impl<T: ItemTemplate> ItemTemplate for &T
• derive macro for ItemTemplate; One might ask, is there a need for macros here?
  • impl<T: ItemTemplate> askama::Template for &T is required
    • Can't apply blanket impl because it involves an external trait
    • So, we are left with implementing this trait for each &T, which can be done by copying the associated types and calling existing methods from T (basically making &T behave the same as T)

I suspect that we can gain more by creating macros here, but it is still under exploration.

cc @GuillaumeGomez

[nicklimmm]

Pulling @notriddle in since the core idea was from him. Feel free to comment :)

I'll be creating the MVP while exploring what can we gain from using macros here.

[nicklimmm]

Posted a draft PR for sharing the proof of concept (mentioned above), looking forward to suggestions :)