org-image-max-width: Allow floating point value as fraction of window width

* lisp/org.el (org--create-inline-image): Allow `org-image-max-width'
to be a floating point value - fraction of window width.
(org-image-max-width): Update the docstring and type specification.

lisp/org.el

@@ -15136,7 +15136,8 @@ t or when #+ATTR* is set to t.
 Possible values:
 - `fill-column' :: limit width to `fill-column'
 - `window'      :: limit width to window width
-- number        :: limit width to number in pixels
+- integer       :: limit width to number in pixels
+- float         :: limit width to that fraction of window width
 - nil             :: do not limit image width"
   :group 'org-appearance
   :package-version '(Org . "9.7")
@@ -15144,7 +15145,8 @@ Possible values:
           (const :tag "Do not limit image width" nil)
           (const :tag "Limit to `fill-column'" fill-column)
           (const :tag "Limit to window width" window)
-          (integer :tag "Limit to a number of pixels")))
+          (integer :tag "Limit to a number of pixels")
+          (float :tag "Limit to a fraction of window width")))
 
 (defcustom org-agenda-inhibit-startup nil
   "Inhibit startup when preparing agenda buffers.
@@ -16285,6 +16287,7 @@ according to the value of `org-display-remote-inline-images'."
                       (`fill-column (* fill-column (frame-char-width (selected-frame))))
                       (`window (window-width nil t))
                       ((pred integerp) org-image-max-width)
+                      ((pred floatp) (floor (* org-image-max-width (window-width nil t))))
                       (`nil nil)
                       (_ (error "Unsupported value of `org-image-max-width': %S"
                                 org-image-max-width)))