doc: fix all headings to use title case

While we hoped to make the headings consistent over time while doing
other edits, we should instead just make the squirrels happy and do them
all at once or they'll likely never be made consistent.

A python script was used to find the headings, and then a call to
https://pypi.org/project/titlecase to transform the title.  A visual
inspection was used to tweak a few unexpected resulting titles.

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>

doc/developer-guides/doc_guidelines.rst

@@ -162,7 +162,7 @@ Would be rendered as:
       Remove all generated output, restoring the folders to a
       clean state.
 
-Multi-column lists
+Multi-Column Lists
 ******************
 
 If you have a long bullet list of items, where each item is short, you
@@ -282,7 +282,7 @@ columns, you can specify ``:widths: 1 2 2``.  If you'd like the browser
 to set the column widths automatically based on the column contents, you
 can use ``:widths: auto``.
 
-File names and Commands
+File Names and Commands
 ***********************
 
 Sphinx extends reST by supporting additional inline markup elements (called
@@ -484,7 +484,7 @@ as needed, generally at least 500 px wide but no more than 1000 px, and
 no more than 250 KB unless a particularly large image is needed for
 clarity.
 
-Tabs, spaces, and indenting
+Tabs, Spaces, and Indenting
 ***************************
 
 Indenting is significant in reST file content, and using spaces is
@@ -641,7 +641,7 @@ without this ``rst-class`` directive will not be numbered.) For example::
 
 .. rst-class:: numbered-step
 
-First instruction step
+First Instruction Step
 **********************
 
 This is the first instruction step material.  You can do the usual paragraphs and
@@ -651,7 +651,7 @@ can move steps around easily if needed).
 
 .. rst-class:: numbered-step
 
-Second instruction step
+Second Instruction Step
 ***********************
 
 This is the second instruction step.