doc: guides: docs: replace diagram image with dot source

Replace the dot diagram with its original source. This is possible
thanks to the Sphinx Graphviz extension. Note that some style attributes
have been removed as defaults are already provided by the extension.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>

doc/guides/docs/images/doc-gen-flow.dot

@@ -1,23 +0,0 @@
-# Doc Generation flow
-# dot -Tpng -odoc-gen-flow.png doc-gen-flow.dot
-
-digraph docgen {
-   node [ fontname="verdana"]
-   bgcolor=transparent; rankdir=LR;
-   images [shape="rectangle" label=".png, .jpg\nimages"]
-   rst [shape="rectangle" label="restructuredText\nfiles"]
-   conf [shape="rectangle" label="conf.py\nconfiguration"]
-   rtd [shape="rectangle" label="read-the-docs\ntheme"]
-   header [shape="rectangle" label="c header\ncomments"]
-   xml [shape="rectangle" label="XML"]
-   html [shape="rectangle" label="HTML\nweb site"]
-   sphinx[shape="ellipse" label="sphinx +\nbreathe,\ndocutils"]
-   images -> sphinx
-   rst -> sphinx
-   conf -> sphinx
-   header -> doxygen
-   doxygen -> xml
-   xml-> sphinx
-   rtd -> sphinx
-   sphinx -> html
-   }

doc/guides/docs/index.rst

@@ -39,8 +39,30 @@ The project's documentation contains the following items:
 * Script-generated material for kernel configuration options based on Kconfig
   files found in the source code tree
 
-.. image:: images/doc-gen-flow.png
-   :align: center
+.. graphviz::
+   :caption: Schematic of the documentation build process
+
+   digraph {
+      rankdir=LR
+
+      images [shape="rectangle" label=".png, .jpg\nimages"]
+      rst [shape="rectangle" label="restructuredText\nfiles"]
+      conf [shape="rectangle" label="conf.py\nconfiguration"]
+      rtd [shape="rectangle" label="read-the-docs\ntheme"]
+      header [shape="rectangle" label="c header\ncomments"]
+      xml [shape="rectangle" label="XML"]
+      html [shape="rectangle" label="HTML\nweb site"]
+      sphinx[shape="ellipse" label="sphinx +\nbreathe,\ndocutils"]
+      images -> sphinx
+      rst -> sphinx
+      conf -> sphinx
+      header -> doxygen
+      doxygen -> xml
+      xml-> sphinx
+      rtd -> sphinx
+      sphinx -> html
+   }
+
 
 The reStructuredText files are processed by the Sphinx documentation system,
 and make use of the breathe extension for including the doxygen-generated API