zfgrep: add page (#8708)

pages/common/zfgrep.md

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+# zfgrep
+
+> Matches fixed strings in possibly compressed files.
+> Equivalent to `grep -F` with input decompressed first if necessary.
+> More information: <https://manned.org/zfgrep>.
+
+- Search for an exact string in a file:
+
+`zfgrep {{search_string}} {{path/to/file}}`
+
+- Count the number of lines that match the given string in a file:
+
+`zfgrep --count {{search_string}} {{path/to/file}}`
+
+- Show the line number in the file along with the matching lines:
+
+`zfgrep --line-number {{search_string}} {{path/to/file}}`
+
+- Display all lines except those that contain the search string:
+
+`zfgrep --invert-match {{search_string}} {{path/to/file}}`
+
+- List only filenames whose content matches the search string at least once:
+
+`zfgrep --files-with-matches {{search_string}} {{path/to/file1 path/to/file2 ...}}`