bug#45435: Additional libraries required by transient and magit manuals

[Jonas Bernoulli <jonas@bernoul.li>]

I wrote magit's info manual using an org file before org-mode itself
switched the input format of its own manual from texinfo to org.  At
the time it was necessary to improve upon "ox-texinfo.el" to make it
suitable for a large manual.

While that may not be the cases anymore, I have kept using my extended
exporter implemented in "ox-texinfo+.el" and would like to keep doing
so.

I am guessing that [non]gnu elpa currently use the version of org that
comes with Emacs.  It would be nice if we could also use the libraries
in org's "contrib/lisp/" directory.

Transient's manual needs "ox-extra.el" library from that directory.
Magit's additionally needs "org-man.el".

"ox-texinfo+.el" is not part of org's "contrib/lisp/" directory and
would have to be added separately to the machines that build the gnu
and nongnu and elpas.

I would need it for the following three features:

;; 1. Create `@deffn' and similar definition items by writing list
;;    items in Org that look similar to what they will look like in
;;    Info.  To enable this, add:
;;
;;      #+TEXINFO_DEFFN: t
;;
;;    to your Org file.  After doing that, you can create definition
;;    items like so:
;;
;;      - Command: magit-section-show
;;
;;        Show the body of the current section.
;;
;;      - Function: magit-git-exit-code &rest args
;;      - Macro: magit-insert-section &rest args
;;      - Variable: magit-display-buffer-noselect
;;      - User Option: magit-display-buffer-function
;;      - Key: q, magit-mode-bury-buffer

;; 2. Optionally share a section's node with some or all of its child
;;    sections.  By default every section on every level gets its own
;;    node, and `ox-texinfo' provides no mechanism for changing that.
;;    To place a section in the same node as its parent section, do
;;    this:
;;
;;      **** Log Performance
;;      :PROPERTIES:
;;      :NONODE: t
;;      :END:

;; 3. Optionally modify the Org file before exporting it.  This is
;;    implemented using a hook that can be set using the `BIND'
;;    property:
;;
;;      #+BIND: ox-texinfo+-before-export-hook some-function
;;      #+BIND: ox-texinfo+-before-export-hook another-function
;;
;;    The function `ox-texinfo+-update-version-strings' is provided
;;    as an example.  It makes some assumptions that might not be
;;    appropriate for your manuals, so you might have to define your
;;    own variant.

(The third feature can also be invoked using a make target and this needs
some love to work properly with "elpa-admin.el", but that's a detail.)

You can use git or a browser to get "ox-texinfo+.el" from
https://github.com/tarsius/ox-texinfo-plus.

Please consider making "org/contrib/lisp/" and "ox-texinfo+.el"
available to the elpas.

     Jonas