docs: Fix RGB565 format documentation and bit masks (#57210)

The RGB565 format documentation in embedder.h incorrectly stated that the red component uses the least significant bits. Unit tests in embedder/testdefs/embedder_unittests.cc demonstrate this is incorrect, showing:

- Red test (0xF800): Uses bits [15:11]
- Green test (0x07E0): Uses bits [10:5]
- Blue test (0x001F): Uses bits [4:0]

This commit fixes the documentation to correctly reflect the actual bit layout:
- Red uses 5 MSBs [15:11]
- Green uses 6 middle bits [10:5]
- Blue uses 5 LSBs [4:0]

Also fixes the example bit extraction code to use correct masks, matching the test expectations.

Impact: This change helps prevent potential developer confusion about RGB565 bit ordering and ensures the documentation matches the actual implementation as verified by the test suite.