[ajnonaka]

Retire the out-of-date Read the Docs site

The docs at https://artemis-em.readthedocs.io were inherited from WarpX and haven't been
maintained. ARTEMIS documentation now lives at https://amrex-microelectronics.github.io/.
This PR turns the Read the Docs site into a single page that redirects there.

Changes

• Removed the old Sphinx content under Docs/source/ (install, usage, theory,
  developers, data analysis, maintenance, LaTeX theory sources, images, bib files, etc.),
  the Doxygen setup (Docs/Doxyfile, Docs/Doxygen/main.dox), Docs/spack.yaml, and
  Docs/README.md.
• Removed the disabled Doxygen CI step from .github/workflows/source.yml and its
  script .github/workflows/source/doxygen. Both referred to the deleted Docs/Doxyfile.
• Docs/source/index.rst: now a single "documentation has moved" page with an
  automatic redirect (<meta http-equiv="refresh">) and a link to the new site.
• Docs/source/conf.py: replaced with a minimal config (built-in alabaster theme,
  no extensions, no Doxygen/breathe, no downloads at build time). It builds only index.rst.
• Docs/requirements.txt: reduced to sphinx>=7.
• Docs/Makefile: replaced the WarpX Makefile (Doxygen, LaTeX and Python build
  targets) with the standard minimal Sphinx Makefile.
• .readthedocs.yml: same build setup, with the WarpX header and htmlzip output removed.

Testing

• cd Docs && pip install -r requirements.txt && make html builds with no warnings. The
  output is a single build/html/index.html that redirects to
  https://amrex-microelectronics.github.io/.

Notes

• No active CI depended on Docs/.
• After merging, a Read the Docs admin may want to deactivate old versions/tags (Admin →
  Versions) so they stop serving the old pages.