Avoid interpreting Liquid in doc pages

They are auto-generated, so they wouldn't normally contain Liquid
(The post-processor may insert some, but it has control over the `raw` tags)
Some code introduced as examples contains `{{` due to nested brace expansions,
which was interpreted as (invalid) Liquid. This fixes such problems.
Additionally, Jekyll generates warnings about excerpts being part of a Liquid
block (the `{% raw %}`). This is fine, since doc pages don't use excerpts.

.github/actions/doc_postproc.awk

@@ -7,6 +7,14 @@
 	sub(/b><\/td/, "th");
 }
 
+# The whole page is being generated, so it's not meant to contain any Liquid
+BEGIN {
+	print "{% raw %}"
+}
+END {
+	print "{% endraw %}"
+}
+
 BEGIN {
 	in_synopsis = 0
 }