[docs] Fix sphinx doc generation errors

Summary:
Errors fixed:
 - GettingStarted: Duplicate explicit target name: "cmake"
 - GlobalISel: Unexpected indentation
 - LoopTerminology: Explicit markup ends without a blank line; unexpected unindent
 - ORCv2: Definition list ends without a blank line; unexpected unindent
 - Misc: document isn't included in any toctree

Verified that a clean docs build (`rm -rf docs/ && ninja docs-llvm-html`) passes with no errors. Spot checked the individual pages to make sure they look OK.

Reviewers: thakis, dsanders

Reviewed By: dsanders

Subscribers: arphaman, llvm-commits, lhames, rovka, dsanders, reames

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66183

llvm-svn: 368932