html: Add support for #+TOC keyword

org mode allows rendering the toc anywhere in the html document using the `TOC`
keyword [1]. There's more options but `#+TOC: headlines $n` should be enough
for starters. Note that org mode still requires setting `#+OPTIONS: toc:nil` to
disable the default toc

[1] https://orgmode.org/manual/Table-of-Contents.html

org/testdata/keywords.pretty_org

@@ -1,4 +1,5 @@
-
+#+OPTIONS: toc:nil
+* captions, custom attributes and more
 #+CAPTION: and _multiple_
 #+CAPTION: lines of *captions*!
 #+ATTR_HTML: :class a b
@@ -23,3 +24,11 @@ named block
 
 # comments must have whitespace after the hashtag
 #not a comment because there's no space after the hashtag
+
+* table of contents
+A table of contents can be rendered anywhere in the document by using
+#+BEGIN_SRC org
+,#+TOC: headlines $n
+#+END_SRC
+Where =$n= is the max headline lvl that will be included. You can use =headlines 0= to include all headlines.
+#+TOC: headlines 0