Update Indentation chapter of Emacs manual.

Update Indentation chapter of Emacs manual.

* indent.texi (Indentation): Rewrite introduction.  Move table to
Indentation Commands node.
(Indentation Commands): Add index entries to table.  Copyedits.
(Tab Stops, Just Spaces): Copyedits.
(Indent Convenience): New node.  Document electric-indent-mode.

* programs.texi (Basic Indent):
* windows.texi (Pop Up Window): Fix kindex entry.