Page version selector and canonical URL should take page aliases into account
Both the page version selector and the canonical URL should take page aliases into account. Currently, if a page is missing for a version, Antora points to the start page for the component. However, a page alias is a perfectly valid target. Therefore, Antora should first look for a page alias if a page is not found before flagging the page as missing.
Here's a summary of the logic:
- Look up the page by ID; if it exists, use it
- Look up the alias by ID (change the family from page to alias); if it exists, use it
- Otherwise use the start page for the component version
The same logic should be used for both the version collection and the canonical URL.
One open question is whether Antora should resolve the alias to a concrete page, or simply link to the URL of the alias. I think I prefer the latter.