doc: make sure section depths are consecutive

skipping heading levels (eg from # to ###, or starting at ###) is legal
in pandoc, but not in nixos-render-docs. pandoc acts as though section
levels *were* consecutive, nixos-render-docs prefers to tell people not
to do that kind of thing because it can make documents more fragile.

doc/hooks/perl.section.md

@@ -1,4 +1,3 @@
-
-### Perl {#setup-hook-perl}
+# Perl {#setup-hook-perl}
 
 Adds the `lib/site_perl` subdirectory of each build input to the `PERL5LIB` environment variable. For instance, if `buildInputs` contains Perl, then the `lib/site_perl` subdirectory of each input is added to the `PERL5LIB` environment variable.