Web: Add introduction to "Download" page
Web: Remove previous warning on "Download" pages

The note about LilyPond being a text based tool has been
replaced by a more explicit introductory section.

I think this makes the warning also obsolete on the subpages
because one will only reach the "Unix" (e.g.) page through the
main "Download" page.
Web: Add introduction to "Editing" page.
Web: Editing: Make Tunefl the last editor

I think Tunefl should get into a new section "Online Tools"

As long as that isn't decided I want to have it out of the way.
Web: Editors: Move LilyPondTool to discontinued

Keep the clickable image although until now there haven't been
images in that section.
But I think that's justified as LilyPondTool is still usable.
Web: Editing: Update Frescobaldi section
Web: Editing: Add Heading
Web: Editing: Rewrite Denemo section