[FIX] doc: fix sphinx warnings

* Since 
https://github.com/odoo/odoo/commit/8d5da6e4be05428f3243f92c0464d3c1f6a43873#diff-e07cbcffb9fb7b2eafd1ace3c4b1626a, 
post_install and at_install test tags have been removed.

* minor warnings in javascript reference.

closes odoo/odoo#39817

Signed-off-by: Victor Feyens (vfe) <vfe@odoo.com>

doc/reference/javascript_reference.rst

@@ -1777,7 +1777,7 @@ order.
       option is useful to inform the web client that the default field name is
       not the name of the current field, but the name of another field.
 
-    - accepted_file_extensions: the file extension the user can pick from the file input dialog box (default value is 'image/*')
+    - accepted_file_extensions: the file extension the user can pick from the file input dialog box (default value is `image/\*`)
       (cf: ``accept`` attribute on <input type="file"/>)
 
     .. code-block:: xml
@@ -2126,6 +2126,7 @@ Relational fields
     - Supported field types: *many2many*
 
     Options:
+
     - accepted_file_extensions: the file extension the user can pick from the file input dialog box
       (cf: ``accept`` attribute on <input type="file"/>)
 

doc/reference/testing.rst

@@ -62,11 +62,16 @@ related to testing Odoo content (modules, mainly):
 
 By default, tests are run once right after the corresponding module has been
 installed. Test cases can also be configured to run after all modules have
-been installed, and not run right after the module installation:
+been installed, and not run right after the module installation::
 
-.. autofunction:: odoo.tests.common.at_install
+  # coding: utf-8
+  from odoo.tests import HttpCase, tagged
 
-.. autofunction:: odoo.tests.common.post_install
+  # This test should only be executed after all modules have been installed.
+  @tagged('-at_install', 'post_install')
+  class WebsiteVisitorTests(HttpCase):
+    def test_create_visitor_on_tracked_page(self):
+        Page = self.env['website.page']
 
 The most common situation is to use
 :class:`~odoo.tests.common.TransactionCase` and test a property of a model