[ADD] extensions/cards: add an extension to implement cards

The extension adds two new directive:
- `cards` is the row container for one or more `card` directives.
- `card` is the implementation of a Bootstrap card that accepts a
  `target` argument for the href of the card, a `large` option to render
  the card on two columns, a `tag` option to display a single arbitrary
  tag on the card, and arbitrary content that is shown in the card
  body.

task-3141419

closes odoo/documentation#3689

X-original-commit: 34bc63d2bc
Signed-off-by: Antoine Vandevenne (anv) <anv@odoo.com>

conf.py

@@ -165,6 +165,9 @@ extensions = [
     # Content tabs
     'sphinx_tabs.tabs',
 
+    # Cards
+    'cards',
+
     # Spoilers
     'spoilers',