Previously, some of the links on the error pages had URLs prefixed with
`https://angular.io/`. This caused them to be treated as external URLs,
which had the following downsides:
- Links would always point to `angular.io` instead of the same version
as the error page (e.g. `next.angular.io` or `v11.angular.io`).
- Dgeni would not be able to check that the URLs are valid (i.e. point
to existing pages).
- An external link icon would incorrectly be shown next to the links on
`angular.io`.
This commit fixes the links to use relative URLs.
PR Close#40881
George Kalpakas
twerske