Changed the method of creating top-level headings

Use an underline only, with no overline on page titles. This makes the
rst include statement easier and more straight forward to use, while
maintaining structure and appearance.

Author: JeffSchering

source/process/sg_document-structure.rst

@@ -1,4 +1,3 @@
-==================
 Document structure
 ==================
 

source/process/sg_grammar-spelling-mechanics.rst

@@ -1,4 +1,3 @@
-================================
 Grammar, spelling, and mechanics
 ================================
 

source/process/sg_mattermost-doc-style.rst

@@ -12,8 +12,5 @@ The Mattermost documentation must be of high quality. It must be accurate and cl
   :backlinks: top
 
 .. include:: sg_document-structure.rst
-  :start-line: 1
 .. include:: sg_grammar-spelling-mechanics.rst
-  :start-line: 1
 .. include:: sg_rest_markup.rst
-  :start-line: 1

source/process/sg_rest_markup.rst

@@ -1,4 +1,3 @@
-=============================
 Using reStructuredText Markup
 =============================
 
@@ -13,15 +12,14 @@ Use the following markup conventions in Mattermost documentation:
 Page titles
 -----------
 
-For page titles, use an overline and an underline. Use the = character. For example:
+For page titles, use the = character for the title underline, with no overline. For example:
 
 .. code-block:: none
   
-  =============================
   Using reStructuredText Markup
   =============================
 
-Unlike Markdown, overlines and underlines in reStructuredText must be at least as long as the title text.
+Unlike Markdown, underlines in reStructuredText must be at least as long as the title text.
 
 Section titles
 --------------