[IMP] odoo_theme: general cleanup

A general reorganization of the code was needed for more clarity.
The different markup styles have been grouped together under a common
"chapter", comments have been rewritten and example pages have been
added for context.

Some levels have been removed Where there were too many for no reason.
By removing one level of styles (e.g.: `#wrap`) we avoid needing to
force overrides or adding `!important` to our code.
⌙ e.g.: before commit 9ef86cd93d8c6fc0fd96b437f629237e7a562c6e,
`.o_spoilers .accordion-body >:first-child` was being involuntarily
overridden.

task-3046383

Part-of: odoo/documentation#3264

extensions/odoo_theme/static/scss/_sphinx_basic.scss

@@ -588,15 +588,15 @@ ol.upperroman {
     list-style: upper-roman;
 }
 
-:not(li) > ol > li:first-child > :first-child,
-:not(li) > ul > li:first-child > :first-child {
-    margin-top: 0px;
-}
+// :not(li) > ol > li:first-child > :first-child,
+// :not(li) > ul > li:first-child > :first-child {
+//     margin-top: 0px;
+// }
 
-:not(li) > ol > li:last-child > :last-child,
-:not(li) > ul > li:last-child > :last-child {
-    margin-bottom: 0px;
-}
+// :not(li) > ol > li:last-child > :last-child,
+// :not(li) > ul > li:last-child > :last-child {
+//     margin-bottom: 0px;
+// }
 
 ol.simple ol p,
 ol.simple ul p,

extensions/odoo_theme/static/scss/_variables.scss

@@ -1,9 +1,3 @@
-$o-grid-breakpoint-xxl: 1400px;
-$o-grid-breakpoint-xxxl: 1680px;
-
-$o-container-max-width-xxl:1380px;
-$o-container-max-width-xxxl: 1560px;
-
 // Typography
 // ===============================================
 $o-font-family-serif:                         'Roboto Slab', Georgia, "Times New Roman", Times, serif;