Use the new section title style in Asciidoctor.

We previous use the section title style like:

Section level 1
===============

Section level 2
---------------

Which have a problem in Asciidoctor that the number of "="s or "-"s must match
the number of characters in the header exactly, as a result it's easy to make
mistakes while changing the titles. Asciidoctor provides a better style like:

= Section level 1

== Section level 2

So we switched to this style.

Also fixed a bug in replace_macros.py, which will not cause any problem in the
old style.

Change-Id: I811dd7238735d98f662767c17086152cd69aea02

Documentation/cmd-version.txt

@@ -1,18 +1,14 @@
-gerrit version
-==============
+= gerrit version
 
-NAME
-----
+== NAME
 gerrit version - Show the version of the currently executing Gerrit server
 
-SYNOPSIS
---------
+== SYNOPSIS
 --
 'ssh' -p <port> <host> 'gerrit version'
 --
 
-DESCRIPTION
------------
+== DESCRIPTION
 Displays a one-line response with the string `gerrit version` followed
 by the currently executing version of Gerrit.
 
@@ -28,16 +24,13 @@ the seven-character abbreviated SHA-1 of the commit. See the `git
 describe` documentation for details on how `<tagname>` is chosen and how
 `<n>` is computed.
 
-ACCESS
-------
+== ACCESS
 Any user who has configured an SSH key.
 
-SCRIPTING
----------
+== SCRIPTING
 This command is intended to be used in scripts.
 
-EXAMPLES
---------
+== EXAMPLES
 
 =====
 	$ ssh -p 29418 review.example.com gerrit version