Clean up docs under install/

This is an initiative to ensure all documentation complies completely
with the Documentation Style Guide section "Documentation is the single
source of truth (SSOT)" according to this epic
https://gitlab.com/groups/gitlab-org/-/epics/1280.

In particular, the changes of this commit touch all files under
doc/install/. Most notably:

- install/redis.md:
        Redirect to Redis section of install guide.

- install/pivotal/index.md:
        Changed its content to reflect the current status, which is no
        longer supported (since 2017!).

- install/google-protobuf.md:
        Incorporated inside installation.md under the troubleshooting section.

- install/openshift_and_gitlab.md:
        Clarified that this guide is no longer relevant. We might want to
        redirect to the Chart docs in the future.

- install/structure.md:
        Incorporated inside installation.md under its own section.

- install/kubernetes/index.md:
        Redirected to https://docs.gitlab.com/charts

1 files changed, 20 insertions, 1 deletions

diff --git a/doc/install/requirements.md b/doc/install/requirements.md
index 4931a69f2a3..a0e719952b4 100644
--- a/doc/install/requirements.md
+++ b/doc/install/requirements.md
@@ -1,5 +1,12 @@
+---
+type: reference
+---
+
 # Requirements
 
+This page includes useful information on the supported Operating Systems as well
+as the hardware requirements that are needed to install and use GitLab.
+
 ## Operating Systems
 
 ### Supported Unix distributions
@@ -12,7 +19,7 @@
 - Scientific Linux (please use the CentOS packages and instructions)
 - Oracle Linux (please use the CentOS packages and instructions)
 
-For the installations options please see [the installation page on the GitLab website](https://about.gitlab.com/installation/).
+For the installations options, see [the main installation page](README.md).
 
 ### Unsupported Unix distributions
 
@@ -225,3 +232,15 @@ Each time a new browser version is released, we begin supporting that version an
 
 NOTE: **Note:** We do not support running GitLab with JavaScript disabled in the browser and have no plans of supporting that
 in the future because we have features such as Issue Boards which require JavaScript extensively.
+
+<!-- ## Troubleshooting
+
+Include any troubleshooting steps that you can foresee. If you know beforehand what issues
+one might have when setting this up, or when something is changed, or on upgrading, it's
+important to describe those, too. Think of things that may go wrong and include them here.
+This is important to minimize requests for support, and to avoid doc comments with
+questions that you know someone might ask.
+
+Each scenario can be a third-level heading, e.g. `### Getting error message X`.
+If you have none to add when creating a doc, leave this section in place
+but commented out to help encourage others to add to it in the future. -->