Put the list of built-in Eshell commands in its own manual node

Put the list of built-in Eshell commands in its own manual node

* doc/misc/eshell.texi (Built-ins): Fix capitalization of node to be
more consistent with the rest of the manual.  Fix a cross reference.
List child nodes.
(List of Built-ins): New section and node.
(Defining New Built-ins): Make this a node.  Fix capitalization.