Update installation doc for 10.1
What does this MR do?
Update installation doc for 10.1 according to https://gitlab.com/gitlab-org/release-tools/blob/master/doc/release-candidates.md#creating-rc1
Update installation doc for 10.1 according to https://gitlab.com/gitlab-org/release-tools/blob/master/doc/release-candidates.md#creating-rc1