Use buck rule for ReleaseNotes instead of Makefile

This eliminates the last Makefile in our code base.

Also change the section style within ReleaseNotes from asciidoc style to
asciidoctor style.

Also I feel that put images/link.png under ReleaseNotes and deal with
all the resource packing is too stupid, so I used the unicode emoji
instead of the picture ("🔗"). If this is too crazy, we can also use "#"
instead :) This also affects documentation rendering.

Other side effects:

1. The css of release notes switched from default asciidoc css into
default asciidoctor css.
2. The section anchors for ReleaseNotes/index.html changed from "2_13"
to "s2_13", because asciidoctorj is unhappy with anchors without
letters.

Change-Id: I4adf2ce090385cc6b699445012f10a009892aaac
diff --git a/ReleaseNotes/ReleaseNotes-2.1.8.txt b/ReleaseNotes/ReleaseNotes-2.1.8.txt
index 476e312..e1ed11c 100644
--- a/ReleaseNotes/ReleaseNotes-2.1.8.txt
+++ b/ReleaseNotes/ReleaseNotes-2.1.8.txt
@@ -1,12 +1,10 @@
-Release notes for Gerrit 2.1.8
-==============================
+= Release notes for Gerrit 2.1.8
 
 Gerrit 2.1.8 is now available:
 
 link:https://www.gerritcodereview.com/download/gerrit-2.1.8.war[https://www.gerritcodereview.com/download/gerrit-2.1.8.war]
 
-New Features
-------------
+== New Features
 * Add cache for tag advertisements
 +
 When READ level access controls are used on references/branches, this
@@ -39,8 +37,7 @@
 MS-DOS compatibility may have permitted access to special device
 files in any directory, rather than just the "\\.\" device namespace.
 
-Bug Fixes
----------
+== Bug Fixes
 * issue 518 Fix MySQL counter resets
 +
 MySQL databases lost their change_id, account_id counters after