Move ISLE docs into cranelift/isle/docs/ and update links.

cranelift/docs/index.md

@@ -14,10 +14,6 @@
  - [Cranelift's register allocator](regalloc.md)
    This page document Cranelift's current register allocator.
    
- - [ISLE](isle.md)
-   This page documents the domain-specific language (DSL), ISLE, that
-   we use to define instruction-lowering patterns.
-
 ## Cranelift crate documentation:
 
  - [cranelift](https://docs.rs/cranelift)

cranelift/isle/README.md

@@ -30,8 +30,10 @@ level, the rules can be seen as simple equivalences between values in two
 languages, and so should be translatable to formal constraints or other logical
 specification languages.
 
-Some more details and motivation are in [BA RFC
-#15](https://github.com/bytecodealliance/rfcs/pull/15). Reference documentation can be found [here](../docs/isle.md).
+Some more details and motivation are in [BA RFC #15](https://github.com/bytecodealliance/rfcs/pull/15).
+Reference documentation can be found [here](docs/language-reference.md).
+Details on ISLE's integration into Cranelift can be found
+[here](../docs/isle-integration.md).
 
 ## Example Usage