docs: split TCG plugin usage from devel section

docs: split TCG plugin usage from devel section

The devel section is getting quite messy with the breakdown of the
example plugins which should be usable by users. As we mention plugins
in the emulation section along with semihosting move the overview
there leaving the development section about the details of writing
plugins.

While we are at make the headings nicer and convert the option lists
into nicely formatted tables.

Acked-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240729144414.830369-11-alex.bennee@linaro.org>