docs: add top level section numbering

Top level sections are not numbered. Adding numbers makes referring to
sections easier. For example the Maintainers page changes from
"about/3.1" to simply "1.3.1".

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: If90a18ee8d6a6858d58f0687f31ea62b69399e04
11 files changed