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/control

@@ -2,7 +2,7 @@ Source: python3-damaris
 Section: science
 Priority: optional
 Maintainer: Markus Rosenstihl <markus.rosenstihl@pkm.tu-darmstadt.de>
-Build-Depends: python3, debhelper, python3-dev, dh-sequence-python3, dpkg-dev, pybuild-plugin-pyproject, sphinx-common, sphinx-rtd-theme-common, python3-sphinxcontrib.bibtex, python3-sphinx-rtd-theme
+Build-Depends: python3, debhelper, python3-dev, dh-sequence-python3, dpkg-dev, pybuild-plugin-pyproject, sphinx-common, python3-sphinx, sphinx-rtd-theme-common, python3-sphinxcontrib.bibtex, python3-sphinx-rtd-theme
 Standards-Version: 3.7.3
 X-Python3-Version: >= 3.5
 Rules-Requires-Root: no
@@ -10,7 +10,7 @@ Rules-Requires-Root: no
 Package: python3-damaris
 Architecture: all
 Provides: damaris-frontend-py3
-Depends: ${python3:Depends}, python3-numpy, python3-scipy, python3-gi, python3-matplotlib, python3-tables, lzop, python3-xdg, libgtksourceview-3.0-1, libgirepository1.0-dev | libgirepository-2.0-dev, breeze-icon-theme
+Depends: ${python3:Depends}, ${sphinxdoc:Depends}, python3-numpy, python3-scipy, python3-gi, python3-matplotlib, python3-tables, lzop, python3-xdg, libgtksourceview-3.0-1, libgirepository1.0-dev | libgirepository-2.0-dev, breeze-icon-theme
 Recommends: damaris-backends, python3-numpy-ext
 Suggests: python3-doc, python3-tables-doc, python3-numpy-doc
 Description: python frontend for DAMARIS project