Simplify doc content reviewer life: warn about aliases when pages have changed

[tbouffard]

In the past, we saw that aliases are often missing. People are not aware that they must configure redirects for moved or deleted pages.
As we did with #373, we should remind/warn contributors when aliases are required.

In the same PR comment we created with #373, we could include a dedicated paragraph about the pages that need an alias.
My proposal is to list the url of former pages, the reviewer then click on the link to check that the redirect is setup (remember that Antora generates redirects that also work in the Surge preview) and goes to the right destination page.

In the future, we could automate that the redirect is correctly setup by running http get commands to at least detect missing alias. But reviewers will always as to check the destination because we cannot automate the check (always specific to the use case)