v1.6: Remove soft deletion

[guimachiavelli]

As part of the current effort in improving indexing performance, the engine team is deprecating soft deletion in favour of other approaches. The documentation makes a few mentions of soft deletion.

Tasks

• Update /learn/inner_workings/storage.mdx

References

• engine issue: About speeding up the indexing processes meilisearch#4071
• SME: @dureuill