docs: issue2551317 add support for jinja2 customization examples Added sphinx-tabs to doc build. This allows adding code-tabs one for "TAL" examples and one for "Jinja2" examples in the doc. Converted partly the "Editing multiple items in an index view" example. Sombody who knows jinja2 better than I, and can test the example will need to finish it. Documented requirement and method for building docs in developers.txt. Added requirements.pip files for pip install -r ... modules needed for processing docs. Fixed missing block in layout.txt that stopped tabs.css from being loaded.

--- a/CHANGES.txt	Tue Feb 13 20:31:42 2024 -0500
+++ b/CHANGES.txt	Sat Feb 17 18:56:04 2024 -0500
@@ -110,6 +110,8 @@
 - issue2551274: add configurable logging for REST API when something
   fails, we now log status code and error message.
   (Ralf Schlatterbeck)
+- issue2551317 - add some Jinja2 examples to customizing.txt
+  document. (John Rouillard)
 
 2023-07-13 2.3.0