[5pt] Release notes for August 2021 releases

[NickVolynkin]

Release notes for August 2021 releases

Product: Tarantool
Root document: https://www.tarantool.io/en/doc/latest/release/
SME: @ Totktonada
Peer reviewer: @ xuniq

Definition of done

• Proofread release notes

[pgulutzan]

@NickVolynkin: two days ago I wrote issue #2296 Behavior changes in SQL. It included suggestions of what should appear in release notes. I see today a new page
https://github.com/tarantool/tarantool/releases/tag/2.10.0-beta1
is different, for example it avoids emphasizing that changes are breaking changes.
If you decide that makes my proposed change obsolete, fine, just say so, but the rest of what I described will still be necessary.