renamed "internals.html" to "index.html"

also, changed all image references to the new PNGs

Note that this document is really out-of-date
(there are no more resources, the stream manager has disappeared,
 the build systel was seriously modified, etc..)

docs/internals/freetype_internals.html

@@ -464,7 +464,7 @@ is possible at run-time, in order to make upgrades easy.</blockquote>
 <blockquote>This diagram show the object relationships for the sole base
 layer. The library object is the root of the object graph :
 <center>
-<p><img SRC="objects_diagram.gif" height=300 width=562></center>
+<p><img SRC="objects_diagram.png" height=300 width=562></center>
 
 <p>It can be read as follows :
 <br>&nbsp;
@@ -575,7 +575,7 @@ with FT_Find_CharMap for a set of given encodings).
 <blockquote>The following diagram illustrates the relationships between
 font objects :
 <center>
-<p><img SRC="objects_diagram2.gif" height=327 width=561></center>
+<p><img SRC="objects_diagram2.png" height=327 width=561></center>
 
 <p>Which can be read as :
 <br>&nbsp;
@@ -704,7 +704,7 @@ object files.
 <br>&nbsp;
 <p><br>
 <center>
-<p><img SRC="build_diagram.gif" height=284 width=559></center>
+<p><img SRC="build_diagram.png" height=284 width=559></center>
 </blockquote>
 
 <h3>