html: Allow limiting level of headlines to be included in toc

The org mode toc OPTION does not just support true/false - it also allows
specifying the max headline level [1] to be included in the toc.

[1] headline level as seen in org mode - not the html tag level

org/testdata/misc.html

@@ -131,6 +131,7 @@ src block
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 <pre>
 #+SETUPFILE: setup_file_org
+#+OPTIONS: toc:1
 * Simple Headline [1/2]
 - [X] checked
 - [ ] unchecked
@@ -163,6 +164,13 @@ By default =EXCLUDE_TAGS= is just =:noexport:=.
 :PROPERTIES:
 not a property
 :END:
+* level limit for headlines to be included in the table of contents
+The toc option allows setting a [[https://orgmode.org/manual/Export-settings.html][level limit]]. For this file we set it to 1 - which means that the following headlines
+won&#39;t be included in the table of contents.
+** headline 2 not in toc
+*** headline 3 not in toc
+** anoter headline 2 not in toc
+you get the gist...
 </pre>
 </div>
 </div>