docs: refresh Tab5 README build guidance

[baba-dev]

Summary

• document ESP-IDF v5.4.2 as the tested toolchain for ESP32-P4 builds
• update the quickstart flow to use platforms/tab5 with the latest idf.py build commands
• remove outdated devcontainer guidance and add golden snapshot maintenance notes

Testing

• not run (docs-only change)

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https://chatgpt.com/codex/tasks/task_e_68ce5d9892cc83249ef3ed10cdc67b4f

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[chatgpt-codex-connector]

[P1] Restore dependency fetch step before building Tab5 firmware

The new quickstart flow no longer fetches the external components that the Tab5 build requires. Following these steps from a fresh clone leaves dependencies/ empty, but platforms/tab5/main/CMakeLists.txt declares mooncake, mooncake_log, and smooth_ui_toolkit in REQUIRES, so idf.py build will fail with missing-component errors. Please reintroduce a step (e.g., python fetch_repos.py) or document another way to populate those dependencies before invoking the build.

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