build HTML docs at package build time and open them from the Help menu

- debian/rules: build sphinx HTML into build/html
- debian/install: install it as usr/share/doc/python3-damaris/html/
  instead of shipping the raw doc/ source tree (whose gitignored
  doc/_build never existed on CI checkouts)
- debian/control: declare ${sphinxdoc:Depends} and python3-sphinx
  (provides sphinx-build) explicitly
- requirements.debian: install python3-sphinx explicitly
- DamarisGUI: point the manual at the packaged HTML path with a
  source-tree fallback, populate the Help dropdown menu, and open
  the DAMARIS manual on a plain toolbar click
- doc/manual.rst: document the Help menu

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

src/damaris/gui/DamarisGUI.py

@@ -1276,15 +1276,31 @@ class DamarisGUI:
 
     def documentation_init( self ):
 
-        self.doc_urls = {
-            "DAMARIS Manual": "file:///usr/share/doc/python3-damaris/doc/_build/html/index.html",
-        }
+        doc_index_candidates = (
+            "/usr/share/doc/python3-damaris/html/index.html",
+            os.path.join( os.path.dirname( os.path.dirname( os.path.dirname(
+                os.path.dirname( os.path.abspath( __file__ ) ) ) ) ),
+                "doc", "_build", "html", "index.html" ),
+        )
+
+        self.doc_urls = { }
+        for doc_index in doc_index_candidates:
+            if os.path.isfile( doc_index ):
+                self.doc_urls[ "DAMARIS Manual" ] = "file://" + doc_index
+                break
 
         if os.path.isdir( "/usr/share/doc/python%d.%d-doc/html" % (sys.version_info[ :2 ]) ):
             self.doc_urls[ "Python Docs" ] = "file:///usr/share/doc/python%d.%d-doc/html/index.html" % (
             sys.version_info[ :2 ])
 
-        doc_index_url = None
+        doc_menu = self.xml_gui.get_object( "documentation_menu" )
+        for doc_title in self.doc_urls:
+            doc_menu_item = gtk.MenuItem( label=doc_title )
+            doc_menu_item.connect( "activate", self.show_doc_menu )
+            doc_menu.append( doc_menu_item )
+            doc_menu_item.show( )
+        self.xml_gui.get_object( "documentation" ).set_menu( doc_menu )
+
         self.doc_browser = None
 
     def show_doc_menu( self, widget, data=None ):
@@ -1294,7 +1310,7 @@ class DamarisGUI:
         if type( widget ) is gtk.MenuItem:
             requested_doc = widget.get_child( ).get_text( )
         else:
-            requested_doc = "Python DAMARIS"
+            requested_doc = "DAMARIS Manual"
 
         if requested_doc in self.doc_urls and self.doc_urls[ requested_doc ] is not None:
             if self.doc_browser is not None: