docs: fix broken Juno links

Certain links to Juno documentation point to a location that were
removed at some point, or are unused. Fix links to point to the latest
available version on Arm's public documentation site, and remove those
that are no longer being used.

Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
Change-Id: I59202767db8834e9c302b2826f3faee47d3a5edd