docs: set up Sphinx/Read the Docs scaffold

[IFX-Anusha]

By creating this pull request you agree to the terms in CONTRIBUTING.md.
https://github.com/Infineon/.github/blob/master/CONTRIBUTING.md
--- DO NOT DELETE ANYTHING ABOVE THIS LINE ---

CONTRIBUTING.md also tells you what to expect in the PR process.

Description

Summary

Sets up a Sphinx / Read the Docs scaffold for this repo, matching the structure used by arduino-core-psoc6.

What's included

• .readthedocs.yaml — RTD build config (Sphinx, docs/conf.py, docs/requirements.txt)
• docs/conf.py — Sphinx config (sphinx_rtd_theme, project "Arduino Core for Zephyr")
• docs/requirements.txt, docs/Makefile, docs/make.bat — build tooling
• docs/index.rst — top-level toctree (CONTENT / CONTRIBUTING sections)
• Stub pages, ready for content: installation-instructions.rst, hw-platforms.rst, arduino-deviations.rst, contributing.rst, env-setup.rst, run-tests.rst

Not included yet

• Page content (currently .. TODO placeholders)
• Board/logo images
• Read the Docs project registration (external, done separately on readthedocs.org)

Verification

• sphinx-build -b html docs docs/_build succeeds cleanly (no warnings/errors)
• Rendered output checked locally: nav, toctree, theme, and footer all correct

[djaumann]

Looks good so far :)