manual: use Atx-style headers except for document title

These are easier to remember than using the Setext-style titles.  Also,
move the Data Layout section to a second-level title from a third-level.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>

doc/manual.adoc

@@ -2,11 +2,9 @@ newfol Manual
 =============
 :doctype: book
 
-Using newfol
-------------
+== Using newfol
 
-Overview
-~~~~~~~~
+=== Overview
 
 newfol is a combination user interface and database store for small databases
 that can fit into memory.  It is designed to represent textual data (stored and
@@ -18,8 +16,7 @@ provides optional support for storing each saved state into Git to preserve a
 complete history of changes.  Along with an SHA-2 hash, this provides immediate
 detection of and recovery from potential data loss situations.
 
-Data Layout
-~~~~~~~~~~~
+== Data Layout
 
 Each newfol database is stored in a directory, which can be specified with the
 `--homedir` option.  The default is `~/.newfol`.
@@ -40,8 +37,7 @@ database where the database contents do not match this file.
 The `lock` file is used to prevent access by multiple processes, which may
 accidentally corrupt each other.
 
-Schema File
-~~~~~~~~~~~
+=== Schema File
 
 The schema file is a CSV (colon-separated values) file.  The first entry in the
 file consist of the values `fmt`, the format version number, and `newfol schema