[UMBRELLA] Define and publish YuniKorn Improvement Proposal (YIP)

In dev mailing list, we have discussed and voted to have YuniKorn Improvement Proposal (YIP).

 
a YIP will define the following parts, including but not limited to:

• what's considered a "major change" that needs a YIP
• what should be included in a YIP (e.g. motivation/business justifications, use case requirements, proposed changes, API changes, migration/compatibility, rejected alternatives, etc)
• who should initiate or be involved in a YIP
• end-to-end process
   
  This is an umbrella and will create subtasks.
   
  We can publish the YIP process to website, and keep all finalized YIP in Confluence. There're projects keeping their XIPs on confluence but I found that is 1) hard to track changes and version control 2) hard to comment or propose changes as not everyone has confluence access . Keep YIP itself on website will solve those issues and make it easier to find.