Fix header query example (#3903)

Co-authored-by: Laurenz <laurmaedje@gmail.com>

crates/typst/src/introspection/query.rs

@@ -13,58 +13,61 @@ use crate::introspection::Location;
 ///
 
 /// # Finding elements
-/// In the example below, we create a custom page header that displays the text
+/// In the example below, we manually create a table of contents instead of
-/// "Typst Academy" in small capitals and the current section title. On the
+/// using the [`outline`] function.
-/// first page, the section title is omitted because the header is before the
-/// first section heading.
 ///
-/// To realize this layout, we open a `context` and then query for all headings
+/// To do this, we first query for all headings in the document at level 1 and
-/// after the [current location]($here). The code within the context block
+/// where `outlined` is true. Querying only for headings at level 1 ensures
-/// runs twice: Once per page.
+/// that, for the purpose of this example, sub-headings are not included in the
+/// table of contents. The `outlined` field is used to exclude the "Table of
+/// Contents" heading itself.
 ///
-/// - On the first page the query for all headings before the current location
+/// Note that we open a `context` to be able to use the `query` function.
-///   yields an empty array: There are no previous headings. We check for this
-///   case and just display "Typst Academy".
-///
-/// - For the second page, we retrieve the last element from the query's result.
-///   This is the latest heading before the current position and as such, it is
-///   the heading of the section we are currently in. We access its content
-///   through the `body` field and display it alongside "Typst Academy".
 ///
 /// ```example
 /// >>> #set page(
-/// >>>   width: 240pt,
+/// >>>  width: 240pt,
-/// >>>   height: 180pt,
+/// >>>  height: 180pt,
-/// >>>   margin: (top: 35pt, rest: 15pt),
+/// >>>  margin: (top: 20pt, bottom: 35pt)
-/// >>>   header-ascent: 12pt,
 /// >>> )
-/// #set page(header: context {
+/// #set page(numbering: "1")
-///   let elems = query(
+///
-///     selector(heading).before(here()),
+/// #heading(outlined: false)[
+///   Table of Contents
+/// ]
+/// #context {
+///   let chapters = query(
+///     heading.where(
+///       level: 1,
+///       outlined: true,
+///     )
 ///   )
-///   let academy = smallcaps[
+///   for chapter in chapters {
-///     Typst Academy
+///     let loc = chapter.location()
-///   ]
+///     let nr = numbering(
-///   if elems.len() == 0 {
+///       loc.page-numbering(),
-///     align(right, academy)
+///       ..counter(page).at(loc),
-///   } else {
+///     )
-///     let body = elems.last().body
+///     [#chapter.body #h(1fr) #nr \ ]
-///     academy + h(1fr) + emph(body)
 ///   }
-/// })
+/// }
 ///
 /// = Introduction
-/// #lorem(23)
+/// #lorem(10)
+/// #pagebreak()
 ///
-/// = Background
+/// == Sub-Heading
-/// #lorem(30)
+/// #lorem(8)
 ///
-/// = Analysis
+/// = Discussion
-/// #lorem(15)
+/// #lorem(18)
 /// ```
 ///
-/// You can get the location of the elements returned by `query` with
+/// To get the page numbers, we first get the location of the elements returned
-/// [`location`]($content.location).
+/// by `query` with [`location`]($content.location). We then also retrieve the
+/// [page numbering]($location.page-numbering) and [page
+/// counter]($counter/#page-counter) at that location and apply the numbering to
+/// the counter.
 ///
 /// # A word of caution { #caution }
 /// To resolve all your queries, Typst evaluates and layouts parts of the