documentation: Added footnote element to the stylesheets

During development, a footnote was added to the Development
manual.  In order to get the footnote to display correctly, I
had to introduce a footnote element in the style sheet. This
commit propogates the style to the other manual CSS sheets.

(From yocto-docs rev: 0439b46620125f2cb26dff4e8c79fc479205133a)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/adt-manual/style.css

@@ -966,3 +966,9 @@ table {
    color: #fff;   
    text-decoration: underline;
 }
+
+.footnote {
+   font-size: small;
+   color: #333;
+}
+

documentation/bsp-guide/style.css

@@ -956,3 +956,9 @@ table {
    color: #fff;   
    text-decoration: underline;
 }
+
+.footnote {
+   font-size: small;
+   color: #333;
+}
+

documentation/kernel-manual/style.css

@@ -966,3 +966,9 @@ table {
    color: #fff;   
    text-decoration: underline;
 }
+
+.footnote {
+   font-size: small;
+   color: #333;
+}
+

documentation/poky-ref-manual/style.css

@@ -966,3 +966,9 @@ table {
    color: #fff;   
    text-decoration: underline;
 }
+
+.footnote {
+   font-size: small;
+   color: #333;
+}
+

documentation/yocto-project-qs/style.css

@@ -966,3 +966,9 @@ table {
    color: #fff;   
    text-decoration: underline;
 }
+
+.footnote {
+   font-size: small;
+   color: #333;
+}
+