index.md: Various improvements

* Remove front-matter
* Better page title
* Use PyMdown admonition format
* Replace raw URLs as links and 'see: <link>' phrasing
* Update URL to Sangoma's phone wiki

Author: seanbright

docs/index.md

@@ -1,17 +1,20 @@
----
-title: Home
-pageid: 4259930
----
+# Asterisk Project Documentation
 
-This is the home of the official documentation for The Asterisk Project.
+This is the home of the official documentation for The Asterisk
+Project.
 
 Please find available content on the left hand menu.
 
-If you would like to make changes or contribute you can find the documentation repo [here](https://github.com/asterisk/documentation).
+If you would like to make changes or contribute you can create
+[a Pull Request](https://github.com/asterisk/documentation/pulls) in
+[our documentation repository on GitHub](https://github.com/asterisk/documentation).
 
 Thank you very much for your continued support of Asterisk!
 
-!!! info
-    Documentation for DAHDI and Digium Asterisk Products is no longer hosted here.  Please see:
-    [https://github.com/asterisk/dahdi-linux/wiki](https://github.com/asterisk/dahdi-linux/wiki)<br>
-    [https://wiki.sangoma.com/display/PHON](https://wiki.sangoma.com/display/PHON)
+/// note | DAHDI And Digium Asterisk Products
+Documentation for DAHDI and Digium Asterisk Products is no longer
+hosted here. There is more information available on
+[the DAHDI Project Wiki](https://github.com/asterisk/dahdi-linux/wiki)
+and
+[Sangoma's Phone Documentation Wiki](https://sangomakb.atlassian.net/wiki/spaces/Phones/overview).
+///