Small changes to Pi Pico related text.

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[what-the-diff]

PR Summary

• Improved Clarity on Supported Architectures
  The description of the architectures that our software supports has been updated. Instead of "RP2040", we now specify "Raspberry Pi Pico". This narrows down the ambiguity for users trying to identify if their systems are compatible.

• Change in Documentation
  The title in the relevant documentation (Sming/Arch/Rp2040/README.rst) has been changed from "Sming RP2040 Architecture" to "Sming Raspberry Pi Pico Architecture". This change ties in with the desire to represent the supported architecture more clearly.

• Expanded Architecture Reference
  In our main reference document (docs/source/index.rst), a more encompassing "Raspberry Pi Pico-series (both RP2040 and RP2350)" has replaced "RP2040". This ensures our users are informed about all the architectures they can work with.