FE part of BE issue 1586 provide index md

* I added an index.md which will be the landing page for the docs. It has an explanation of Pleroma-FE from the user point of view
* See also BE MR: pleroma!2669
* And issue: pleroma#1586

docs/USER_GUIDE.md

@@ -8,8 +8,6 @@
 >
 > --Catbag
 
-Pleroma-FE user interface is modeled after Qvitter which is modeled after older Twitter design. It provides a simple 2-column interface for microblogging. While being simple by default it also provides many powerful customization options.
-
 ## Posting, reading, basic functions.
 
 After registering and logging in you're presented with your timeline in right column and new post form with timeline list and notifications in the left column.

docs/index.md

+# Introduction to Pleroma-FE
+## What is Pleroma-FE?
+
+Pleroma-FE is the default user-facing frontend for Pleroma. It's user interface is modeled after Qvitter which is modeled after an older Twitter design. It provides a simple 2-column interface for microblogging. While being simple by default it also provides many powerful customization options.
+
+## How can I use it?
+
+If your instance uses Pleroma-FE, you can acces it by going to your instance (e.g. <https://pleroma.soykaf.com>). You can read more about it's basic functionality in the [Pleroma-FE User Guide](./USER_GUIDE.md). We also have [a guide for administrators](./CONFIGURATION.md) and for [hackers/contributors](./HACKING.md).