[IMP] conf.py, *: interpolate the version and releases in the content

This commit replaces hard-coded occurrences of the version and of the
last, current and next releases' version with placeholders interpolated
at build time to avoid manually updating these after each freeze.

task-2917614

closes odoo/documentation#3166

X-original-commit: 12e1085199
Signed-off-by: Antoine Vandevenne (anv) <anv@odoo.com>

content/developer/howtos/rdtraining/B_acl_irrules.rst

@@ -12,10 +12,10 @@ ACL stands for "Access Control List"
     <howto/rdtraining>`.
 
     To follow the exercise, it is recommended that you fetch the branch
-    15.0-core from the
-    `technical training solutions <https://github.com/odoo/technical-training-solutions/tree/15.0-core>`__ repository.
-    It contains a version of the module created during the core training we can use
-    as a starting point.
+    {BRANCH}-core from the `technical training solutions
+    <https://github.com/odoo/technical-training-solutions/tree/{BRANCH}-core>`_ repository. It
+    contains a version of the module created during the core training we can use as a starting
+    point.
 
 So far we have mostly concerned ourselves with implementing useful features.
 However in most business scenarios *security* quickly becomes a concern: