templates: Introduce template framework for examples and tests

We want each example and test to work on all supported reference
platforms (currently Corstone-300) as well as RTOSes that are compatible
with any given example. Also, we would like each example to work like a
typical user project which targets one particular platforms and one
particular RTOS (or none, if the app runs on bare metal). This creates a
large matrix of examples x platforms x RTOSes (or baremetal).

To avoid having to maintain all possible combinations, we introduce a
mechanism for generating examples (similarly for tests). See updated
documentation for how to use this mechanism.

When the main branch is updated (i.e. an MR is merged), publish newly
generated examples to the repository that hosts full, ready-to-use
examples for users' convenience.

In CI runs, all flavors (RTOSes) of the same example are grouped into
the same pipeline to minimize the number of required runners, while
different platforms and toolchains are put into their own pipelines
as before because each of our Docker images contains only one toolchain
or FVP.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>