[MLGO][Docs] Add initial skeleton of MLGO docs

llvm/docs/MLGO.rst

@@ -0,0 +1,28 @@
+====
+MLGO
+====
+
+Introduction
+============
+
+MLGO is a framework for integrating ML techniques systematically in LLVM. It is
+designed primarily to replace heuristics within LLVM with machine learned
+models. Currently there is upstream infrastructure for the following
+heuristics:
+
+* Inlining for size
+* Register allocation (LLVM greedy eviction heuristic) for performance
+
+This document is an outline of the tooling that composes MLGO.
+
+Corpus Tooling
+==============
+
+..
+    TODO(boomanaiden154): Write this section.
+
+Model Runner Interfaces
+=======================
+
+..
+    TODO(mtrofin): Write this section.

llvm/docs/Reference.rst

@@ -41,6 +41,7 @@ LLVM and API reference documentation.
    PDB/index
    PointerAuth
    ScudoHardenedAllocator
+   MLGO
    MemoryModelRelaxationAnnotations
    MemTagSanitizer
    Security