doc: libvlc: add main page for libVLC documentation

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

doc/standalone/libvlc.md

+# LibVLC Documentation {#libvlc_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#.
+
+The generated documentation can be browsed from [here](@ref libvlc).
+
+## License
+
+[libVLC](@ref libvlc_documentation) is 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>.
+
+## Example
+
+\snippet{lineno} example.c minimal example