Convert cross references properly for code content

[dandhlee]

Currently cross references are only converted to xref tags and are not usable within code content. For example:

>>> import <xref uid="bigframes.pandas">bigframes.pandas</xref> as bpd
>>> bpd.options.display.progress_bar = None

We'll need to find direct links, however should be possible. See go examples: https://cloud.google.com/go/docs/reference/cloud.google.com/go/alloydb/latest/apiv1#cloud_google_com_go_alloydb_apiv1_DefaultAuthScopes