duperemove: add page (#4231)

pages/linux/duperemove.md

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+# duperemove
+
+> Finds duplicate file system extents and optionally schedule them for deduplication.
+> An extent is small part of a file inside the file system.
+> On some file systems one extent can be referenced multiple times, when parts of the content of the files are identical.
+> More information: <https://markfasheh.github.io/duperemove/>.
+
+- Search for duplicate extents in a directory and show them:
+
+`duperemove -r {{path/to/directory}}`
+
+- Deduplicate duplicate extents on a Btrfs or XFS (experimental) file system:
+
+`duperemove -r -d {{path/to/directory}}`
+
+- Use a hash file to store extent hashes (less memory usage and can be reused on subsequent runs):
+
+`duperemove -r -d --hashfile={{path/to/hashfile}} {{path/to/directory}}`
+
+- Limit I/O threads (for hashing and dedupe stage) and CPU threads (for duplicate extent finding stage):
+
+`duperemove -r -d --hashfile={{path/to/hashfile}} --io-threads={{N}} --cpu-threads={{N}} {{path/to/directory}}`