Doc: Fix internal links in interrupt framework design guide

Some of the sections numbers have been incorrect since the document
structure has changed. Using references to sections rather than
hard-coded section numbers fixes the issue while making it more
future-proof to updates.

Change-Id: I80f5ab6a55a0073b1562a02ce2c9ec7d67cddfff
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>