docs: add available social networks to the guide

docs/update_rendercv_files.py

@@ -177,6 +177,12 @@ def define_env(env):
     themes = [f"`{theme}`" for theme in dm.available_themes]
     env.variables["available_themes"] = ", ".join(themes)
 
+    # available social networks strings (put available social networks between ``)
+    social_networks = [
+        f"`{social_network}`" for social_network in dm.available_social_networks
+    ]
+    env.variables["available_social_networks"] = ", ".join(social_networks)
+
 
 def generate_entry_figures():
     """Generate an image for each entry type and theme."""

docs/user_guide/structure_of_the_yaml_input_file.md

@@ -53,13 +53,15 @@ cv:
   phone: +905419999999
   website: https://example.com/
   social_networks:
-    - network: LinkedIn
+    - network: LinkedIn # (1)!
       username: yourusername
     - network: GitHub 
       username: yourusername
   ...
 ```
 
+1.  The available social networks are: {{available_social_networks}}. You can add more social networks by following the same pattern. The social network icons are automatically added to the header of the CV.
+
 None of the values above are required. You can omit any or all of them, and RenderCV will adapt to your input. These generic fields are used in the header of the CV.
 
 The main content of your CV is stored in a field called `sections`.