doc/sphinx: Add doxygen aliases for easier rst embedding

With these aliases we can use rST directives inside comment blocks. This
adds also a doxygen command '\rststar' where ignores the asterisk --
typical to multi-line comment blocks.

While at it, add a simple example on how to use them.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

doc/sphinx/doxygen.ini.in

@@ -238,7 +238,10 @@ TAB_SIZE               = 8
 # "Side Effects:". You can put \n's in the value part of an alias to insert
 # newlines.
 
-ALIASES                =
+ALIASES = "rst=\verbatim embed:rst"
+ALIASES += "endrst=\endverbatim"
+ALIASES += "rststar=\verbatim embed:rst:leading-asterisk"
+ALIASES += "endrststar=\endverbatim"
 
 # This tag can be used to specify a number of word-keyword mappings (TCL only).
 # A mapping has the form "name=value". For example adding "class=itcl::class"