docs: Move bootindex.txt into system section and rstify

docs: Move bootindex.txt into system section and rstify

Move bootindex.txt into the system section of the manual and turn it
into rST format.  To make the document make more sense in the context
of the system manual, expand the title and introductory paragraphs to
give more context.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 20210727194955.7764-1-peter.maydell@linaro.org