Help: Simplify top-level index organization

Revise the organization introduced in commit 2c7cd95c (Help: Organize
top-level index, 2013-10-28) to drop "Other Manuals" and put them in
"Reference Manuals" because the distinction between them has blurred.
Perhaps a better breakdown of the reference manuals will emerge in
the future.

While at it, sort the reference manual toctree by name.

Help/index.rst

@@ -26,24 +26,17 @@ Reference Manuals
    :maxdepth: 1
 
    /manual/cmake-commands.7
+   /manual/cmake-developer.7
+   /manual/cmake-generator-expressions.7
    /manual/cmake-generators.7
-   /manual/cmake-toolchains.7
+   /manual/cmake-language.7
    /manual/cmake-modules.7
    /manual/cmake-packages.7
    /manual/cmake-policies.7
    /manual/cmake-properties.7
+   /manual/cmake-toolchains.7
    /manual/cmake-variables.7
 
-Other Manuals
-#############
-
-.. toctree::
-   :maxdepth: 1
-
-   /manual/cmake-developer.7
-   /manual/cmake-generator-expressions.7
-   /manual/cmake-language.7
-
 .. only:: html
 
  Index and Search