docs/devel: add some front matter to the devel index

Give an overview of the most useful bits of the devel documentation to
read depending on what the developer wants to do.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230630180423.558337-23-alex.bennee@linaro.org>
4 files changed