docs: patch Poetry build instructions

docs: patch Poetry build instructions

Some parts of the documentation referring to Poetry provides incorrect
build instructions and has some minor formatting errors. Reformat the
bits that require formatting, and fix the build instructions. These
were originally part of the patch stack that added Poetry support but
were accidentally reverted prior to merge.

Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
Change-Id: I336d3a7bbe99f75262430ae436f8ebc2cb050d2c