[IMP] upgrade-util: link code documentation

Add direct code documentation from upgrade-util repo.
Adapt `extensions/github_link`.

Original commit: 3352d33997

X-original-commit: 26fd5f3192
Part-of: odoo/documentation#8849
Co-authored-by: Victor Feyens <vfe@odoo.com>

conf.py

@@ -133,6 +133,24 @@ else:
         )
         odoo_dir_in_path = True
 
+if odoo_dir_in_path:
+    upgrade_util_dir = next(filter(Path.exists, [Path('upgrade-util'), Path('../upgrade-util')]), None)
+    if not upgrade_util_dir:
+        _logger.warning(
+            "Could not find Upgrade Utils sources directory in `upgrade_util`.\n"
+            "The developer documentation will be built but autodoc directives will be skipped.\n"
+            "In order to fully build the 'Developer' documentation, clone the repository with "
+            "`git clone https://github.com/odoo/upgrade-util` or create a symbolic link."
+        )
+        odoo_dir_in_path = False
+    else:
+        _logger.info(
+            "Found Upgrade Util sources in %(directory)s",
+            {'directory': upgrade_util_dir.resolve()},
+        )
+        from odoo import upgrade
+        upgrade.__path__.append(str((upgrade_util_dir / 'src').resolve()))
+
 # Mapping between odoo models related to master data and the declaration of the
 # data. This is used to point users to available xml_ids when giving values for
 # a field with the autodoc_field extension.
@@ -250,6 +268,9 @@ redirects_dir = 'redirects/'
 sphinx_tabs_disable_tab_closing = True
 sphinx_tabs_disable_css_loading = True
 
+# Autodoc ordering
+autodoc_member_order = 'bysource'
+
 #=== Options for HTML output ===#
 
 html_theme = 'odoo_theme'