add porting in a nutshell car analogy (#157)

* add porting in a nutshell car analogy

* rephrase intro sentence to use 'porting'

* remove portin in a nutshell from first steps

* add porting in a nutshell to start page and fix smaller wording errors

Author: Danfro

index.rst

@@ -2,17 +2,21 @@
 Halium
 ======
 
-Welcome to the documentation for `Halium`_, the collaborative project to unify the Hardware Abstraction Layer for projects which run GNU/Linux on mobile devices with pre-installed Android.
+Welcome to the `Halium`_ porting guide. Halium is a collaborative project to unify the Hardware Abstraction Layer for projects which run GNU/Linux on mobile devices with pre-installed Android.
+
+What is porting about - in a nutshell
+-------------------------------------
+Think of your device as a car. You have got a petrol car (Android). You want to put in an electric driven engine (target OS). In the garage you are told you can put in a diesel engine (e.g. Lineage OS). Fine, so your car is capable of exchanging the engine. Now you need to get all parts for your new engine (kernel, firmware, ...), assemble them and build them into your car (porting). And because every car's chassis is different, you can not use the parts of a Ford to use with a Porsche. That's why every type of car needs its own engine (port). Got it?
 
 Contents
 ========
 
-   
+
 .. toctree::
    :maxdepth: 1
    :caption: Porting Guide
    :name: sec-porting
-   
+
    porting/first-steps
    porting/get-sources
    porting/build-sources
@@ -23,24 +27,24 @@ Contents
    :maxdepth: 1
    :caption: Distribution
    :name: sec-distribution
-   
+
    Distribution
-   
+
 .. toctree::
    :maxdepth: 1
    :caption: Supplementary Information
    :name: sec-supplemental
-   
+
    supplementary/about-this-chapter
    supplementary/devices/index
    supplementary/hardware-enablement
    supplementary/how-to-document
-   
+
 .. toctree::
    :maxdepth: 1
-   :caption: Project 
+   :caption: Project
    :name: sec-planning
-   
+
    project/Scope
    project/Related-projects
    project/Planning