Add "Editing" page/node (issue 60530047)

[lilyliska]

Reviewers: ,

Message:
This has been waiting for some time, but is in line with stuff I did
earlier and was also discussed earlier.

A new page/node making the relation between input file, _any_ editor and
_dedicated_ editor more explicit, hopefully being a step to reduce
confusion for potential new users.

Description:
Add "Editing" page/node




Web:Text input: Remove LilyKDE image in closing

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And it's very outdated BTW (a discontinued program).

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.

Web:Introduction: Add link to new Editing node


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.

Web: Add node "Editing"

As discussed on lilypond-devel I wrote a page discussing the concept
of editing and compiling itself.
While this factually is self-evident it seems that we need this
extra effort in getting potential users on the right track.

Please review this at https://codereview.appspot.com/60530047/

Affected files (+98, -13 lines):
  M Documentation/web/introduction.itexi