[IMP] odoo_theme: add default BS functions

Prior to this change it was not possible to use Bootstrap default SCSS
functions in 'bootstrap_overridden.scss'.

This commit mirrors odoo framework's architecture, injecting bootstrap
'_functions.scss' at the beginning of the documentation's SCSS "bundle".

Commit preparatory to the introduction of the 'Spoiler' documentation
component.

task-3036845

Part-of: odoo/documentation#2837

extensions/odoo_theme/static/style.scss

@@ -1,3 +1,4 @@
+@import "scss/bootstrap_scss/_functions.scss";
 @import "scss/_sphinx_basic";
 @import "scss/_variables";
 @import "scss/bootstrap_overridden";