Although the current documentation exposes only limited functionality,
which is unlikely to change, add an explicit note that the build system
v2 is currently a technical preview intended for testing and early
evaluation.
Ensure consistency in referring to v2 throughout the documentation. In
some places, it is referred to as V2 or Version 2; unify this to v2.
Additionally, update the hello_world example for v2 to use the
idf::spi_flash alias instead of the interface variable to maintain
consistency with the rest of the documentation.
Signed-off-by: Frantisek Hrbata <frantisek.hrbata@espressif.com>
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