doc: improve consistency in documentation titles

Unify capitalization and spaces and add some info for consistency,
clarity and coherence with sorting in the all.md / _toc.md.

PR-URL: https://github.com/nodejs/node/pull/11230
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

doc/api/addons.md

@@ -1,4 +1,4 @@
-# Addons
+# C/C++ Addons
 
 Node.js Addons are dynamically-linked shared objects, written in C or C++, that
 can be loaded into Node.js using the [`require()`][require] function, and used

doc/api/net.md

@@ -1,4 +1,4 @@
-# net
+# Net
 
 > Stability: 2 - Stable
 

doc/api/process.md

@@ -1,4 +1,4 @@
-# process
+# Process
 
 <!-- type=global -->
 

doc/api/punycode.md

@@ -1,4 +1,4 @@
-# punycode
+# Punycode
 
 > Stability: 0 - Deprecated
 

doc/api/string_decoder.md

@@ -1,4 +1,4 @@
-# StringDecoder
+# String Decoder
 
 > Stability: 2 - Stable
 

doc/api/util.md

@@ -1,4 +1,4 @@
-# util
+# Util
 
 > Stability: 2 - Stable
 

doc/api/vm.md

@@ -1,4 +1,4 @@
-# Executing JavaScript
+# VM (Executing JavaScript)
 
 > Stability: 2 - Stable