Document const bindings

In some use cases of `bindInstance`, people might want to bind constants. To support this use case, the following changes are needed:

* Change the normalization of `const C`, `const C&` and `const C*` to be `const C` (instead of `C`)
* Forbid binding both `const C` and `C` (with tests)
* When a `const C` binding is required, a `C` binding can satisfy this requirement (also add tests for this)
* Add tests for `bindInstance(const C&)`
* Document the caveat of `bindInstance(const C&)` (the reference must outlive the injector) and add a deleted `bindInstance(C&&)` overload to make sure temporaries are rejected.
* Update comments and wiki to mention the new kind of binding, add examples to the reference documentation