Fixed broken external links

Related to: https://gitlab.com/gitlab-org/technical-writing/-/issues/492

doc/administration/raketasks/maintenance.md

@@ -115,7 +115,7 @@ component servers like [Gitaly](../gitaly/configure_gitaly.md#run-gitaly-on-its-
 You may also have a look at our troubleshooting guides for:
 
 - [GitLab](../index.md#troubleshooting)
-- [Omnibus GitLab](https://docs.gitlab.com/omnibus/README.html#troubleshooting)
+- [Omnibus GitLab](https://docs.gitlab.com/omnibus/index.html#troubleshooting)
 
 To run `gitlab:check`, run:
 

doc/integration/index.md

@@ -89,7 +89,7 @@ at Super User also has relevant information.
 
 **Omnibus Trusted Chain**
 
-[Install the self signed certificate or custom certificate authorities](https://docs.gitlab.com/omnibus/common_installation_problems/README.html#using-self-signed-certificate-or-custom-certificate-authorities)
+[Install the self signed certificate or custom certificate authorities](https://docs.gitlab.com/omnibus/common_installation_problems/index.html#using-self-signed-certificate-or-custom-certificate-authorities)
 in to Omnibus GitLab.
 
 It is enough to concatenate the certificate to the main trusted certificate

doc/raketasks/index.md

@@ -12,7 +12,7 @@ common administration and operational processes.
 
 You can perform GitLab Rake tasks by using:
 
-- `gitlab-rake <raketask>` for [Omnibus GitLab](https://docs.gitlab.com/omnibus/README.html)
+- `gitlab-rake <raketask>` for [Omnibus GitLab](https://docs.gitlab.com/omnibus/index.html)
   installations.
 - `bundle exec rake <raketask>` for [source](../install/installation.md)
   installations.