fadvise: add page (#15365)

Co-authored-by: Sebastiaan Speck <12570668+sebastiaanspeck@users.noreply.github.com>
Co-authored-by: Wiktor Perskawiec <git@spageektti.cc>

pages/linux/fadvise.md

@@ -0,0 +1,16 @@
+# fadvise
+
+> Control Linux file caching behavior.
+> More information: <https://manned.org/fadvise>.
+
+- Preload a file into cache:
+
+`fadvise {{-a|--advice}} willneed {{path/to/file}}`
+
+- Suggest dropping a file from cache:
+
+`fadvise {{path/to/file}}`
+
+- Display help:
+
+`fadvise --help`