(docs): export: add snippet for backlinks with contents. (#466)

doc/org_export.md

@@ -1,6 +1,10 @@
-To include the backlinks in your Org file export -- whether using Org's
+While exporting your documents to another format such as HTML -- whether using Org's in-built export or ox-hugo -- you can add a backlinks section which would display the backlinks to the current file. This is done via org-export's preprocessing hooks. See more at [Advanced Export Configuration (The Org Manual)](https://orgmode.org/manual/Advanced-Export-Configuration.html#Advanced-Export-Configuration).
-in-built publishing or ox-hugo -- use the following snippet to add a
+
-"Backlinks" section at the end of the page:
+Following are two different configs that might be suitable for different use-cases. Add one of these snippets in your Emacs init file.
+
+### Only Backlinks
+
+This will display only the backlinks and not the contents.
 
 ```emacs-lisp
 (defun my/org-roam--backlinks-list (file)
@@ -21,3 +25,37 @@ in-built publishing or ox-hugo -- use the following snippet to add a
 
 (add-hook 'org-export-before-processing-hook 'my/org-export-preprocessor)
 ```
+
+### Backlinks and Contents
+
+This would insert both backlinks and the contents, just like the org-roam buffer. This might be especially useful if you host a web version of your personal knowledgebase and want to browse the files along with the backlinks from mobile devices.
+
+```emacs-lisp
+(defun my/org-roam--backlinks-list-with-content (file)
+  (with-temp-buffer
+    (if-let* ((backlinks (org-roam--get-backlinks file))
+              (grouped-backlinks (--group-by (nth 0 it) backlinks)))
+        (progn
+          (insert (format "\n\n* %d Backlinks\n"
+                          (length backlinks)))
+          (dolist (group grouped-backlinks)
+            (let ((file-from (car group))
+                  (bls (cdr group)))
+              (insert (format "** [[file:%s][%s]]\n"
+                              file-from
+                              (org-roam--get-title-or-slug file-from)))
+              (dolist (backlink bls)
+                (pcase-let ((`(,file-from _ ,props) backlink))
+                  (insert (s-trim (s-replace "\n" " " (plist-get props :content))))
+                  (insert "\n\n")))))))
+    (buffer-string)))
+
+  (defun my/org-export-preprocessor (backend)
+    (let ((links (my/org-roam--backlinks-list-with-content (buffer-file-name))))
+      (unless (string= links "")
+        (save-excursion
+          (goto-char (point-max))
+          (insert (concat "\n* Backlinks\n") links)))))
+
+  (add-hook 'org-export-before-processing-hook 'my/org-export-preprocessor)
+```