Overridable `debuginfo` type (#945)

This change enables the override of the auto-detected
`/etc/os-release`-based `debuginfo_type` attribute of the
`build_repo_for_rpmbuild_toolchain` repository rule and allows the
auto-detection by means of a new `default` option.

This also enables the use of` none` as an override option.

This change also includes a regeneration of the documentation, which
seems to have trailed recent code changes. But there are no
documentation changes related to this change as
`build_repo_for_rpmbuild_toolchain` repository rule is not documented
(yet).