Recognize .zst-compressed man pages + fix SQL basename_from_filename() to recognize .xz

Also greatly simplified basename_from_filename() because apparently I
couldn't write regexes back then.

(And the removed REFERENCES line is to sync schema.sql with the actual
state of the DB, which doesn't have that constraint for some reason.
I'll prolly fix that later)

indexer/src/man.rs

@@ -27,7 +27,7 @@ pub fn parse_path(path: &str) -> Option<(&str, &str, &str)> {
             /man[a-z0-9]/      # Subdir
             ([^/]+?)           # Man page name (non-greedy)
             \. ([^/\.]+)       # Section
-            (?: \. (?: gz|lzma|bz2|xz ))* $  # Any number of compression extensions
+            (?: \. (?: gz|lzma|bz2|xz|zst ))* $  # Any number of compression extensions
         ").unwrap();
     }