Document how to deal with major changes on the source – PG, OS, glibc, etc. upgrades, reconfiguration, etc (DLE)

Create a HowTo for admins describing how to deal with various changes – such as PG upgrades, backup location change, etc. Separately for "physical" and "logical" modes. What needs to be done on DLE side, how to re-initiate data retrieval without downtime, etc.

Special attention to:

• physical backup location changes
• WAL-G, pgBackRest, etc. versions and problem of "forward compatibility"
• glibc and possible index corruption