docs: Reorganize landing page.

I clustered the docs by some intuitive/fuzzy notion of "similarity", and
some reasonable categories seemed to materialize. I tried to give the
clusters useful names, but you may want to take a look at the landing
page (<http://clang.llvm.org/docs/> for the lazy) and share your
thoughts.

I have to say, this small change really gives the docs a whole new life!
It makes our documentation quite a bit easier to navigate and scope out.

llvm-svn: 171379

clang/docs/index.rst

@@ -9,29 +9,59 @@ Welcome to Clang's documentation!
 .. toctree::
    :maxdepth: 1
 
-   LanguageExtensions
-   LibASTMatchers
-   LibTooling
-   PCHInternals
-   Tooling
+   ReleaseNotes
+
+Using Clang as a Compiler
+-------------------------
+
+.. toctree::
+   :maxdepth: 1
+
+   UsersManual
    AddressSanitizer
    ThreadSanitizer
    MemorySanitizer
+
+Using Clang as a Library
+------------------------
+
+.. toctree::
+   :maxdepth: 1
+
+   Tooling
+   IntroductionToTheClangAST
+   LibTooling
    ClangPlugins
+   RAVFrontendAction
+   LibASTMatchersTutorial
+   LibASTMatchers
    ClangTools
    HowToSetupToolingForLLVM
-   IntroductionToTheClangAST
    JSONCompilationDatabase
-   LibASTMatchersTutorial
-   PTHInternals
-   RAVFrontendAction
-   UsersManual
-   AutomaticReferenceCounting
+
+Design Documents
+----------------
+
+.. toctree::
+   :maxdepth: 1
+
    InternalsManual
    DriverInternals
-   BlockLanguageSpec
+   PTHInternals
+   PCHInternals
+
+
+Language Extensions and Specs
+-----------------------------
+
+.. toctree::
+   :maxdepth: 1
+
+   LanguageExtensions
    ObjectiveCLiterals
-   ReleaseNotes
+   BlockLanguageSpec
+   AutomaticReferenceCounting
+
 
 Indices and tables
 ==================