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>

requirements.debian

@@ -1,7 +1,7 @@
 #!/bin/bash
 apt-get -y install libcairo2-dev gir1.2-gtksource-3.0 hdf5-tools
 apt-get -y install python3-dev python3-scipy python3-lmfit python3-matplotlib python3-tables python3-xdg  python3-setuptools python3-sphinx-rtd-theme
-apt-get -y install debhelper dh-sequence-python3 lsb-release sphinx-common pybuild-plugin-pyproject python3-sphinxcontrib.bibtex
+apt-get -y install debhelper dh-sequence-python3 lsb-release sphinx-common python3-sphinx pybuild-plugin-pyproject python3-sphinxcontrib.bibtex
 DEBRELEASE=$(lsb_release -cs)
 case $DEBRELEASE in
   "trixie")