pg_combinebackup: add page (#18737)

Co-authored-by: Managor <42655600+Managor@users.noreply.github.com>
Co-authored-by: Dylan <145150333+dmmqz@users.noreply.github.com>

pages/common/pg_combinebackup.md

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+# pg_combinebackup
+
+> Reconstruct a full (synthetic) PostgreSQL backup from an incremental backup chain.
+> When specifying multiple backups order them from oldest to newest.
+> More information: <https://www.postgresql.org/docs/current/app-pgcombinebackup.html>.
+
+- Combine a full and incremental backup into one synthetic full backup:
+
+`pg_combinebackup {{path/to/full_backup}} {{path/to/incremental_backup}} {{[-o|--output]}} {{path/to/output_directory}}`
+
+- Perform a dry run to show what would be done, without creating files:
+
+`pg_combinebackup {{[-n|--dry-run]}} {{path/to/full_backup}} {{path/to/incremental_backup}} {{[-o|--output]}} {{path/to/output_directory}}`
+
+- Use hard links instead of copying files (faster, same filesystem required):
+
+`pg_combinebackup {{[-k|--link]}} {{path/to/full_backup}} {{path/to/incremental_backup}} {{[-o|--output]}} {{path/to/output_directory}}`
+
+- Use file cloning (reflinks) for efficient copy if supported:
+
+`pg_combinebackup --clone {{path/to/full_backup}} {{path/to/incremental_backup}} {{[-o|--output]}} {{path/to/output_directory}}`
+
+- Use the `copy_file_range` system call for efficient copying:
+
+`pg_combinebackup --copy-file-range {{path/to/full_backup}} {{path/to/incremental_backup}} {{[-o|--output]}} {{path/to/output_directory}}`
+
+- Relocate a tablespace during reconstruction:
+
+`pg_combinebackup {{path/to/backup1 path/to/backup2 ...}} {{[-T|--tablespace-mapping]}} /{{path/to/old_tablespace}}=/{{path/to/new_tablespace}} {{[-o|--output]}} {{path/to/output_directory}}`
+
+- Disable fsync for faster but unsafe writes (testing only):
+
+`pg_combinebackup {{[-N|--no-sync]}} {{path/to/backup1 path/to/backup2 ...}} {{[-o|--output]}} {{path/to/output_directory}}`
+
+- Show detailed debug output:
+
+`pg_combinebackup {{[-d|--debug]}} {{path/to/backup1 path/to/backup2 ...}} {{[-o|--output]}} {{path/to/output_directory}}`