doc: add main page for documentation

Small main page for describing the content of the documentation, while
splitting and describing the role of both libVLC and libvlccore.

It is ought to be extended later.

Co-authored-by: Simon Latapie <garf@videolabs.io>

doc/standalone/mainpage.md

@@ -0,0 +1,24 @@
+\mainpage
+# LibVLC and libvlccore documentation
+
+## Introduction
+
+[libVLC](@ref libvlc_documentation) is an embeddable engine for
+3rd party applications and frameworks.
+
+It runs on the same platforms as VLC <em>(and sometimes on more)</em>
+and can provide playback, streaming and conversion of multimedia
+files and streams.
+
+[libVLC](@ref libvlc_documentation) has numerous bindings for
+other languages, such as C++, Python, java, Objective-C and C#.
+
+##  License
+
+[libVLC](@ref libvlc_documentation) and **libvlccore** are released
+under the LGPLv2 <em>(or later)</em> license. This allows embedding the
+engine in 3rd party applications, while letting them to be licensed
+under other licenses.
+
+However, note that some plugins are under more restrictive licenses,
+such as GPLv3 <em>(or later)</em>.