Align website claims and quickstart with current Open4D implementation

[cicm4]

Website corrections required before campaign use

The site at https://open4dfoundation.github.io/ should not be a primary
campaign destination until it reflects the current repository.

High-risk mismatches

• python examples/play_mesh_o4d.py does not exist. The current viewer is
  examples/visualization/visualize_sequence.py.
• The site describes .o4d, open4d.io, open4d.metrics, stable public APIs,
  writers, and tools that the implementation-status page explicitly marks as
  unimplemented roadmap work.
• “From clone to playback in three commands” omits the [player] extra and a
  real input path.
• The site presents integrated, reproducible codec modules and benchmarks more
  strongly than the repository audit supports.
• It calls April 2026 a core API release and June 2026 an N4MC/TSMC integration,
  while the current audit says the codecs remain mostly isolated from the
  shared model.
• It encourages installation without the repository's redistribution and
  third-party provenance warning.

Safe replacement claims

• “Research workspace for time-varying 3D geometry.”
• “Under active development; shared interfaces are evolving.”
• “Includes a lightweight temporal geometry core, examples/viewers, adapters,
  and independent codec and reconstruction research pipelines.”
• “Clone from source and use the versioned handbook for verified status.”

Safe quickstart

git clone https://github.com/open4dfoundation/Open4D.git
cd Open4D
python3 -m venv .venv
source .venv/bin/activate
python -m pip install -e '.[player]'
python examples/visualization/visualize_sequence.py path/to/frames --info

The campaign should switch its primary destination to the website only after
these claims and commands are corrected and tested.