Docs: double check the links to the about website install page

Per gitlab-com/www-gitlab-com#3206 (closed)

Update all links as follows

Old links to	Should be updated to
https://about.gitlab.com/installation/	https://about.gitlab.com/install/
https://about.gitlab.com/installation/ce-or-ee/	https://about.gitlab.com/install/ce-or-ee/

Occurrences

CE

doc/downgrade_ee_to_ce/README.md:84:[CE installation instructions](https://about.gitlab.com/installation/?version=ce).                                                                                           
doc/university/support/README.md:48:- [Installation Methods](https://about.gitlab.com/installation/):
doc/intro/README.md:44:- [Install GitLab](https://about.gitlab.com/installation/)
doc/development/code_review.md:271:our [Omnibus packages](https://about.gitlab.com/installation/), but some use                                                                                                  
doc/administration/index.md:16:You can [install GitLab CE or GitLab EE](https://about.gitlab.com/installation/ce-or-ee/),                                                                                        
doc/install/google_cloud_platform/index.md:54:1. Next, follow the instructions for installing GitLab for the operating system you choose, at https://about.gitlab.com/installation/. You can use the IP address from the step above, as the hostname.
doc/install/requirements.md:15:For the installations options please see [the installation page on the GitLab website](https://about.gitlab.com/installation/).                                                   
doc/install/requirements.md:26:Please see the [installation from source guide](installation.md) and the [installation guides](https://about.gitlab.com/installation/) for more information.                      
doc/install/README.md:49:[methods]: https://about.gitlab.com/installation/

EE

doc/downgrade_ee_to_ce/README.md:84:[CE installation instructions](https://about.gitlab.com/installation/?version=ce).
doc/university/support/README.md:48:- [Installation Methods](https://about.gitlab.com/installation/):
doc/intro/README.md:44:- [Install GitLab](https://about.gitlab.com/installation/)
doc/development/code_review.md:271:our [Omnibus packages](https://about.gitlab.com/installation/), but some use
doc/administration/geo/replication/security_review.md:118:  [GitLab installation](https://about.gitlab.com/installation/) page for more
doc/administration/geo/replication/index.md:163:1. [Install GitLab Enterprise Edition](https://about.gitlab.com/installation/) on the server that will serve as the **secondary** node. Do not create an account or log in to the new **secondary** node.
doc/administration/index.md:16:You can [install GitLab CE or GitLab EE](https://about.gitlab.com/installation/ce-or-ee/),
doc/install/google_cloud_platform/index.md:54:1. Next, follow the instructions for installing GitLab for the operating system you choose, at https://about.gitlab.com/installation/. You can use the IP address from the step above, as the hostname.
doc/install/requirements.md:15:For the installations options please see [the installation page on the GitLab website](https://about.gitlab.com/installation/).
doc/install/requirements.md:26:Please see the [installation from source guide](installation.md) and the [installation guides](https://about.gitlab.com/installation/) for more information.
doc/install/README.md:51:[methods]: https://about.gitlab.com/installation/