cmd/integration: add mdbx_copy compact in-place one-liner (#21189)

Adds a `## Compact chaindata in-place` section to
`cmd/integration/Readme.md` with the `mdbx_copy -c -u` one-liner that
compacts the DB in-place using a temp file, then atomically renames it
over the original.

Author: AskAlexSharov

cmd/integration/Readme.md

@@ -164,6 +164,12 @@ rm -rf datadir/snapshots/*borch*
 erigon snapshots integrity --datadir /erigon-data/ --check=BorCheckpoints
 ```
 
+## Compact chaindata in-place
+
+```sh
+src=<datadir>/chaindata && ./build/bin/mdbx_copy -c -u "$src" "${src}/mdbx.dat.tmp" && mv "${src}/mdbx.dat.tmp" "${src}/mdbx.dat" || rm -f "${src}/mdbx.dat.tmp"
+```
+
 ## See tables size
 
 ```sh