doc/ivy.org: Improve the markup

doc/ivy.org: Improve the markup

* doc/ivy.org (defopt): Get rid of the macro.
(Writing this manual): New section.

Use a lot more definition lists - they look nice in both HTML and
Texinfo export.
(Hydra in the minibuffer): Extend the section.
(Saving the current completion session to a buffer): Extend.

* doc/ivy-ox.el (org-texinfo-plain-list): Improve the user-option
  formatting.