document delegation tokens

Got some questions today about delegation tokens. The asker referenced this very old page from the old HBase wiki (which we deprecated as defunct ages ago):

https://wiki.apache.org/hadoop/Hbase/HBaseTokenAuthentication

I was going to refer them to the up to date docs, since I thought we migrated all the content. However, a review of the current Ref Guide shows that we do not have docs on delegation tokens.

Ideally this would be 2 sections:

• an addition to the "secure configuration" section that explained how to use delegation tokens in a secure cluster, with the primary example of their use in MR and/or spark
• an appendix that explains the current implementation for those working on hbase internals