[Config] Document the 'database.hot_standby' option

**Product:** Tarantool
**Since:** 3.0
**Root document:** https://www.tarantool.io/en/doc/latest/reference/configuration/configuration_reference/#configuration-reference-database-hot-standby
**SME:** @ totktonada

# Details

Old content: https://www.tarantool.io/en/doc/latest/reference/configuration/#hot-standby
Dev issue: https://github.com/tarantool/tarantool/issues/775
