fc*: move to common (#6510)

pages/common/fc-cache.md

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+# fc-cache
+
+> Scan font directories to build font cache files.
+> More information: <https://manned.org/fc-cache>.
+
+- Generate font cache files:
+
+`fc-cache`
+
+- Force a rebuild of all font cache files, without checking if cache is up-to-date:
+
+`fc-cache -f`
+
+- Erase font cache files, then generate new font cache files:
+
+`fc-cache -r`