Definitively fix blank page issues

Introduce a slightly altered version of the old page break logic,
but with a small tweak so that the `avoid` only applies to the very
first page, and not to nested sections. Thus there is no page break
before the TOC, but there is always a page break before a section,
even when nested.

Author: Bittor Alaña

src/mkdocs_print_site_plugin/css/print-site.css

@@ -96,9 +96,12 @@ Print site table of contents styling
     #print-page-toc { display: block }
 
     /* PDF page breaks on each MkDocs page, except the first one */
-    #print-site-page section.print-page:not(:has(> #print-page-toc)) {
+    #print-site-page section.print-page {
         page-break-before: always;
     }
+    #print-site-page > section.print-page:first-of-type {
+        page-break-before: avoid;
+    }
 
     /* PDF page breaks - separate title page for each section */
     #print-site-page section.print-page.md-section > h1 {