[doc] How to add interrupts to IP
[doc] How to add interrupts to IP

This commit adds a section in Getting Started HW.
The section describes how-to add/revise the interrupts.
Currently, the designer needs to revise many parts (DV, SW, unittest)
when updating the interrupts.

Co-Authored-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>
Signed-off-by: Eunchan Kim <cb7bc7ceb22698f298c67faa994fd9ddf3cab298@opentitan.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan