Reuse content with conref and conkeyref

Write a note, step or paragraph once in a chunks file and pull it into any topic. The build inlines it, and every page that uses it updates together.

Put reusable elements, each with an id, in a topic under reuse/.

<topic id="common">
  <title>Reusable content</title>
  <body>
    <note id="restart" type="note">Restart the web service and the worker after changing a variable.</note>
    <steps>
      <step id="step-build"><cmd>Build the packages: <codeph>pnpm build</codeph>.</cmd></step>
    </steps>
  </body>
</topic>

Leave the file out of every map. A chunks file is not published on its own, and validation does not warn about it as long as some published topic uses one of its chunks.

Pull a chunk in with conref, using the path, the topic id and the element id.
```
<note conref="../../reuse/common.dita#common/restart"/>
```
Or define a key for the chunks file once, and use conkeyref so topics need no relative paths.
```

<keydef keys="reuse" href="../reuse/common.dita"/>


<step conkeyref="reuse/step-build"/>
```
This page itself pulls its restart note in with conkeyref:

Note: The server reads its settings once, at start-up. After you change a variable, restart (or redeploy) both the web service and the worker.

Results

How references behave:

References work within a file and across files, and chunks may reference other chunks. Cycles are reported as conref-cycle.
The referencing element should have the same name as the chunk. A <p> that pulls a <note> works but warns (conref-type-mismatch).
Filtering attributes on the referencing element apply to the pulled content, so <note conkeyref="reuse/restart" product="core"/> shows the shared note to core readers only. Attributes on the chunk itself apply too.
Keys inside a chunk resolve in the keyspace of the map publishing the topic that pulls it.
conrefend and conaction are not supported: only the start element is pulled, with a warning.

The release snapshot records which topics use each chunk, so tools can show the impact of changing one.