🗃️(backend) export to docx

We can now export our document to a docx file.
This is done by converting the html to a docx
file using the pypandoc and pandoc library.
We added the "format" param to the
generate-document endpoint, "format" accept
"pdf" or "docx" as value.

src/backend/demo/data/template/code.txt

@@ -1,7 +1,7 @@
 <page size="A4">
   <div class="header">
-    <img
-      src="https://upload.wikimedia.org/wikipedia/fr/7/72/Logo_du_Gouvernement_de_la_R%C3%A9publique_fran%C3%A7aise_%282020%29.svg"
+    <img width="200"
+      src="https://impress-staging.beta.numerique.gouv.fr/assets/logo-gouv.png"
     />
   </div>
   <div class="content">

src/backend/demo/data/template/css.txt

@@ -1,18 +1,20 @@
 body {
   background: white; 
-  font-family: arial
-}
-.header {
-  display: flex;
-  justify-content: space-between;
+  font-family: arial;
 }
 .header img {
   width: 5cm;
   margin-left: -0.4cm;
 }
 .body{
-  margin-top: 1.5rem
+  margin-top: 1.5rem;
 }
 img {
   max-width: 100%;
 }
+[custom-style="center"] {
+  text-align: center;
+}
+[custom-style="right"] {
+  text-align: right;
+}