Release Notes as a separate document

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Release notes and headline features for 2.3

Release notes for 2.3 This section will be updated prior to the next release. Headline features Release Notes Deprecations Upgrade warnings Index and column named digest on SQL tables messages and sources will be dropped during db:migrate step and replaced by more effective hash index on column named value. While we are working on redesigning how reports are stored in the SQL database, hash indices are faster and use significantly less memory storage than btree in this case. Our tests indicate…