docs: populate installation, boards, contributing, env-setup, run-tests

[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.

Summary

Populates the Read the Docs scaffold (from ifx-readthedocs-setup) with real content for the KIT-PSE84-AI board, based on psoc-edge-main's existing install/pinout docs and the arduino-core-tests integration work.

Refer here - https://arduinocore-zephyr.readthedocs.io/en/ifx-docs-content/index.html

Pages populated

• Installation Instructions — Arduino IDE Boards Manager steps and Arduino CLI install/flash/compile/upload commands, using the real infineon:zephyr_pse84 package index URL (verified against edge-main's actual package/package_infineon_pse84_index.template.json).
• Supported Boards — KIT-PSE84-AI FQBN, silicon (PSE846GPS2DBZC4A), pinout image, and pin-macro conventions (Dx/Ax ranges, LED_BUILTIN*, BTN_BUILTIN, SERIAL_INTx) — all cross-checked against the current variant.h on edge-main.
• Contributing — fork/branch/commit/PR flow plus the <scope>: <description> commit/PR title convention.
• Environment Setup — prerequisites, bootstrap script, build/flash the loader, install arduino-cli, install the core, local-checkout development via symlink, compile/upload.
• Running Tests — how tests/test_config.h board blocks work, with the KIT-PSE84-AI test_digitalio_single example (D0/D1 jumper) and the ENABLE_SYNC gotcha.

Intentionally excluded

• Internal Infineon branch-topology/PR-targeting policy (ifx-main/edge-main/control-main, rebase policy) — team-internal process, not relevant to external readers.
• Detailed OpenOCD/WSL/udev/PAT-token troubleshooting — kept only the one common first-time blocker (~/.west/config OpenOCD path), rest better suited to an internal wiki.
• Arduino API Deviations — left as TODO, deferred until peripheral support is further along.

Verification

• sphinx-build -b html docs docs/_build succeeds cleanly (no warnings/errors).
• Every factual claim (FQBN, package index naming, pin macros, script names/usage) was cross-checked directly against the current edge-main source, not just copied from the reference material.

Note

run-tests.rst documents the tests/arduino_core_tests submodule and tests/test_config.h, which aren't merged into ifx-main yet (still on ifx-arduino-core-tests/edge-arduino-core-tests-config). The page is accurate for once those land.