adding file extension that is optional for Antora 2.x, but will soon be required with Antora 3.x.

Also adding an explicit anchor to avoid breaking anchors when headings change.

https://docs.antora.org/antora/3.0/whats-new/#antora-3-0-0-alpha-1

Antora has added the .adoc file extension to a page coordinate in page aliases and xrefs whenever it wasn’t specified by the writer. This fallback mechanism has been deprecated in Antora 3.0 to make way for using non-AsciiDoc pages in the xref facility. Review the page IDs in your xrefs and page-aliases attributes to ensure the .adoc extension is specified before upgrading.