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>

debian/install

@@ -1,4 +1,4 @@
-doc/ usr/share/doc/python3-damaris/
+build/html usr/share/doc/python3-damaris/
 debian/damaris3.desktop usr/share/applications/
 src/damaris/gui/DAMARIS3.png usr/share/pixmaps/
 src/damaris/gui/DAMARIS3.ico usr/share/pixmaps/