Issue 3845: Web: Rewrite Closing of "Text input"

Discussion indicated that "Text input" - while very good in
itself - isn't sufficient to let all readers get the idea
of "editing and compiling".
Therefore I added a new page discussing that concept.

This commit rewrites the closing of "Text input" to make
that clear and @ref to the new page.
Issue 3845: Web:Text input: Remove LilyKDE image

As there is a dedicated page about editing now I think this
image here isn't needed anymore.
There are already so many nice images on that page.
And it's very outdated BTW (a discontinued program).
Issue 3845: Color coding for "Introduction" menu
Issue 3845: Add images to Documentation/pictures

English and German versions, small and full-size.
Material to generate a different version and
translations are available in lilypond-extra
Issue 3845: Web:Editing: Add new flow-chart

Attention: This image isn't present in lilypond-extra yet.
So outside of my dev computer this won't compile correctly.
Issue 3845: Web:Intro: Add link to new Editing node
Issue 3845: Fix Whitespace errors