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>

doc/manual.rst

@@ -101,6 +101,12 @@ There are now two possibilities to export the figure:
 2. Saving the data of the plot as CSV file (comma separated values) by pressing the button on the right side of the toolbar under the plot
 
 
+Help Menu
+=========
+
+The **Help** button in the toolbar opens this documentation in the system's web browser. The arrow next to it offers additional local documentations, such as the Python documentation, if installed. When the Debian package is used, the HTML documentation is found in ``/usr/share/doc/python3-damaris/html/``.
+
+
 Accumulation
 ============