docs: Improve docs on running examples

Move the section `CMake presets` closer to `Examples` and `Tests` and
note that the presets are only available when the SDK is the top-level
project, useful when building examples and tests.

Document where to find a test executable in the build directory, and
remind users to look at an example's own `README.md` (if one exists)
for additional parameters to pass to the FVP (e.g. to enable network).

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