fix(docs-infra): add the Exported by section to the API table of contents - #71145

…ents

`SectionDescription` writes a bare `<h2>Exported by</h2>` instead of using
`SectionHeading`, so it is the only heading the API templates emit without an
id. Across the built API pages there are 1023 `<h2 id="api">`, 336
`<h2 id="description">`, 225 `<h2 id="usage-notes">`, 13 `<h2 id="pipe-usage">`
and 61 bare `<h2>`, all of them this one.

API detail pages render a table of contents, and the loader selects `h2[id]`,
so the section is missing from the ToC on all 61 pages, including every pipe
and every `@ngModule`-tagged directive. It also has no anchor, so it cannot be
linked to.

Using `SectionHeading` gives it the `exported-by` id, the anchor link and the
section margin the other headings get. The `<hr>` above it is dropped: it was
the only rule in the API templates and stood in for the margin this heading
never had, so keeping both left a rule and a 3rem gap doing the same job.

`exported-by` is added to `KNOWN_API_SECTION_ANCHORS`, which the file asks to
be kept in sync with the `SectionHeading` usages, so a JSDoc link to that
fragment validates.