docs: getting_started.md and for_spectre_users.md list diode and psp103 as bundled but they require vendor git submodules

[github-actions]

Files: `docs/getting_started.md` (line 211), `docs/for_spectre_users.md` (line 64-79)

Problem:

`docs/getting_started.md` line 211 states:

Bundled models — VAJAX ships with common models (resistor, capacitor, diode, PSP103, BSIM-BULK, BSIM-CMG, HiSIM2, and more)

`docs/for_spectre_users.md` also lists `diode` and `psp103` under "Bundled Models".

Reality: Inspecting `vajax/analysis/openvaf_models.py`:

MODEL_PATHS = {
    "psp103": ("integration_tests", "PSP103/psp103.va"),  # from vendor/OpenVAF/
    "diode": ("vacask", "diode.va"),                      # from vendor/VACASK/
    ...
    "resistor": ("bundled", "resistor.va"),               # truly bundled
    "capacitor": ("bundled", "capacitor.va"),             # truly bundled
}

• `psp103` is loaded from `vendor/OpenVAF/integration_tests/PSP103/psp103.va`
• `diode` is loaded from `vendor/VACASK/devices/diode.va`

These vendor paths are git submodules NOT included in the PyPI sdist (`pyproject.toml` sdist includes only `vajax/devices/models/**`). Users who `pip install vajax` cannot use these models.

The docs also reference `vendor/VACASK/benchmark/` circuits (line 153, 171 of getting_started.md) which are similarly unavailable to pip install users.

Fix: Clarify that `diode` and `psp103` require a source install (git clone with submodules), or move their source files into the `vajax/devices/models/` bundled directory and include them in the package.