doc: ti: add the K3 generation page

Texas Instrument's entire K3 generation of SoCs use much of the same
frameworks and boot flow, especially at the uboot level. Though there
are small differences introduced as each new K3 based SoC is developed
and as the K3 generation matures that will also need to be documented.

Rather than copying the same documentation, with the small differences
applicable to that specific SoC to a new page, introduce a new K3
page that can describe the general boot flow and design decisions for
the entire K3 generation of chips, leaving the specifics for that
particular SoC to a unique sub-page below this one.

Signed-off-by: Bryan Brattlof <bb@ti.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>