Clean & document layout

[echebbi]

There's a lot of stuff here and I believe we can make it clearer for newcomers. At first sight I have the following comments:

Directory	Comment
archive/	What's this? Is it still relevant? We have a SCM so "archived" stuff could just be removed from the current branch.
examples/	Some ALE examples along with compiled Java code. A bit old; they likely need an update & some comments to help understanding
languages/	Languages implemented with ALE along with different compilation targets... Only for testing purposes? Aren't they also examples?
programs/	AST serialized as XMI. What are they for? Who should use them? How?

Moreover it looks like plugins/org.eclipse.emf.ecoretools.ale.compiler.app is actually an Eclipse application that depends on languages/minijava.lang.services to compile a MiniJava program; it should likely be moved to examples/ or languages/.

@dvojtise some p2 update sites are hardcoded in POM:

ale-lang-compiler/pom.xml

Lines 29 to 40 in bede815

	<repositories>
	<repository>
	<id>acceleo</id>
	<url>http://download.eclipse.org/acceleo/updates/releases/</url>
	<layout>p2</layout>
	</repository>
	<repository>
	<id>ale-lang-repo</id>
	<url>http://www.kermeta.org/ale-lang/updates/2019-11-26/</url>
	<layout>p2</layout>
	</repository>
	</repositories>

Is there any reason not to put them in the target platform?