docs: add hotlinks to about preface text

docs: add hotlinks to about preface text

Make it easier to navigate the documentation.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230124180127.1881110-20-alex.bennee@linaro.org>