Add ISLE reference documentation.

This documentation provides details for all of the ISLE language
features, and detailed rationale for why many of them are designed in
the way that they are. It is hopefully both a reasonable tutorial and
reference for someone looking to understand the DSL.

Note that this documentation is separate from and orthogonal to the
work to document the Cranelift bindings and integration work that
@fitzgen has covered well in #3556. This document can link to that one
and vice-versa once they are both in-tree.

cranelift/docs/index.md

@@ -13,6 +13,10 @@
 
  - [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: