Revise documentation (incl. alignment with NEST simulator)

[jhnnsnk]

Current preview: https://jhnnsnknest-gpu.readthedocs.io

• Align overall file structure with NEST simulator, e.g., from doc/ to doc/htmldoc, and update build instructions, e.g., requirements.txt
• Align overall appearance, e.g., using sphinx_material theme, and adjust style files; also add favicon
• New NEST GPU-specific banner (network to "GPU cluster" transition) with Python script for generation and checked-in .svg file
• Organise Navigation into USAGE, COMMUNITY, and RELATED PROJECTS
• Landing page with new introductory text and Highlights carousel
• Add refs.bib as bibliography for the whole documentation
• Add new page "Publications" that shows each NEST GPU paper with its citation, a teaser figure and some key points
• Add a new guide on Connectivity concepts building on the NEST simulator version
• Revise installation instructions
• Add new page "Cite NEST GPU"
• Add new page "Contact us and contribute" emphasizing joint development with NEST simulator; replace old content partially from current version of NEST simulator
• Add new page "What's new?" linking to the GitHub release notes
• RELATED PROJECTS now shows NEST simulator and NESTML (the functionality to generate neuron models with NESTML is also mentioned in the Model Directory and the guide for implementing new neuron models)
• Add license
• Add 404 not found page

(coding partially assisted by Claude)