Help: Reorganize install() sections

After switching to the signature directive we no longer need
per-signature sections.  Group all the signatures in one section.  Add a
table of contents since it is no longer redundant with the synopsis.

Help/command/install.rst

@@ -1,6 +1,10 @@
 install
 -------
 
+.. only:: html
+
+   .. contents::
+
 Specify rules to run at install time.
 
 Synopsis
@@ -107,8 +111,8 @@ signatures that specify them.  The common options are:
   they will be created according to the uname rules on Unix-like platforms.
   Windows platforms are unaffected.
 
-Installing Targets
-^^^^^^^^^^^^^^^^^^
+Signatures
+^^^^^^^^^^
 
 .. signature::
   install(TARGETS <target>... [...])
@@ -475,9 +479,6 @@ Installing Targets
     to ensure that such out-of-directory targets are built before the
     subdirectory-specific install rules are run.
 
-Installing Imported Runtime Artifacts
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
 .. signature::
   install(IMPORTED_RUNTIME_ARTIFACTS <target>... [...])
 
@@ -515,9 +516,6 @@ Installing Imported Runtime Artifacts
   added to the ``<set-name>`` runtime dependency set. This set can then be
   installed with an :command:`install(RUNTIME_DEPENDENCY_SET)` command.
 
-Installing Files
-^^^^^^^^^^^^^^^^
-
 .. signature::
   install(FILES <file>... [...])
   install(PROGRAMS <program>... [...])
@@ -620,9 +618,6 @@ Installing Files
     use "generator expressions" with the syntax ``$<...>``.  See the
     :manual:`cmake-generator-expressions(7)` manual for available expressions.
 
-Installing Directories
-^^^^^^^^^^^^^^^^^^^^^^
-
 .. signature::
   install(DIRECTORY <dir>... [...])
 
@@ -760,9 +755,6 @@ Installing Directories
     The list of ``dirs...`` given to ``DIRECTORY`` may use
     "generator expressions" too.
 
-Custom Installation Logic
-^^^^^^^^^^^^^^^^^^^^^^^^^
-
 .. signature::
   install(SCRIPT <file> [...])
   install(CODE <code> [...])
@@ -800,9 +792,6 @@ Custom Installation Logic
     name, not the file's contents).  See the
     :manual:`cmake-generator-expressions(7)` manual for available expressions.
 
-Installing Exports
-^^^^^^^^^^^^^^^^^^
-
 .. signature::
   install(EXPORT <export-name> [...])
 
@@ -910,9 +899,6 @@ Installing Exports
     :command:`install_files`, and :command:`install_programs` commands
     is not defined.
 
-Installing Runtime Dependencies
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
 .. signature::
   install(RUNTIME_DEPENDENCY_SET <set-name> [...])