spec: align IDL-Motoko.md with Float32 support (#6094)

## Summary

- Adds `e(Float32) = float32` and `i(float32) = Float32` to the type
mapping tables.
- Removes the note that said importing `float32` was
unsupported/failing.

## Why

`Float32` support was shipped in #5906, which updated both
`idl_to_mo.ml` and `mo_to_idl.ml` to handle the `float32 ↔ Float32`
mapping. The spec doc (`design/IDL-Motoko.md`) was not updated at the
time, leaving it contradicting the implementation.

This is a companion to a fix in
[dfinity/candid](https://github.com/dfinity/candid/pull/new/fix/motoko-binding-float32)
where `didc bind --target mo` panics on any `.did` file containing
`float32`, because it (correctly) follows the now-outdated spec.

Made with [Cursor](https://cursor.com)

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: Kamirus

design/IDL-Motoko.md

@@ -85,6 +85,7 @@ e(Nat) = nat
 e(Int) = int
 e(Nat<n>) = nat<n> for n = 8, 16, 32, 64
 e(Int<n>) = int<n> for n = 8, 16, 32, 64
+e(Float32) = float32
 e(Float) = float64
 e(Char) = nat32
 e(Text) = text
@@ -135,7 +136,7 @@ i(nat) = Nat
 i(int) = Int
 i(nat<n>) = Nat<n> for n = 8, 16, 32, 64
 i(int<n>) = Int<n> for n = 8, 16, 32, 64
-// i(float32) not defined
+i(float32) = Float32
 i(float64) = Float
 i(text) = Text
 i(reserved) = Any
@@ -256,9 +257,6 @@ escape_method <name> = (* failure, unsupported *)
    ```
    In other words: Motoko subtyping must be contained in IDL subtyping.
 
- * There is no way to produce `float32`.
-   Importing interfaces that contain `float32` types fails.
-
  * The functions `escape`/`unescape` ensure round-tripping of IDL field names
    through Motoko types. See `IDL-Motoko.proofs.md` for details.