Restructure Eshell extension modules documentation

Restructure Eshell extension modules documentation

This adds a section for documenting all the optional modules.

Do not merge to master.  This is a backport of f2981a1681d.

* doc/misc/eshell.texi (Extension modules): Move explanation about
writing modules to...
(Writing a module): ... here.
(Module testing): Remove.  Testing an Eshell module doesn't require
any special documentation.
(Key binding, Smart scrolling, Electric forward slash): Move under...
(Optional modules): ... here.
(Directory handling, Terminal emulation): Remove.  These modules are
enabled by default, and so are documented above.
(Tramp extensions, Extra built-in commands): New sections.