[globalisel] Restructure the GlobalISel documentation

There's a couple minor deletions amongst this but 99% of it is just moving
the documentation around to prepare the way for more meaningful changes.
diff --git a/llvm/docs/Reference.rst b/llvm/docs/Reference.rst
index 3fbceba..4c421a20 100644
--- a/llvm/docs/Reference.rst
+++ b/llvm/docs/Reference.rst
@@ -23,7 +23,7 @@
    FuzzingLLVM

    GarbageCollection

    GetElementPtr

-   GlobalISel

+   GlobalISel/index

    GwpAsan

    HowToSetUpLLVMStyleRTTI

    HowToUseAttributes

@@ -125,7 +125,7 @@
    A reference manual for the MIR serialization format, which is used to test

    LLVM's code generation passes.

 

-:doc:`GlobalISel`

+:doc:`GlobalISel/index`

   This describes the prototype instruction selection replacement, GlobalISel.

 

 =====================

@@ -208,4 +208,4 @@
   LLVM support for coroutines.

 

 :doc:`YamlIO`

-   A reference guide for using LLVM's YAML I/O library.
\ No newline at end of file
+   A reference guide for using LLVM's YAML I/O library.