Add SGR-ISGW Compatibility statement

Include a section in ISGR docs to describe compatibility of nodes using SGR and ISGR.
The PRD has a table under compatibility section https://docs.google.com/document/d/13E6JOq8u_AaUd_t8FZEPCAuq7jfkjryecjBQBHE3pG8/edit?disco=AAAAIAHAIa4&usp=todo_email_discussion&usp_dm=false&ts=6046a7e4
 
Extend the upgrade notes on ISGR - we should  include a section under our global Upgrade page- You can add a link to the ISGR section if you prefer to not move it. (edited)