Make clearer that the memory team is working
to reduce the memory requirement

I'm adding some wording to our hardware requirements to explain that needs vary
by workload. I also included some examples of factors that may influence the
workload.

Update the Unicorn worker documentation based on the new scaling
formula.

https://gitlab.com/gitlab-org/omnibus-gitlab/issues/4535

* Use `GiB memory` to describe memory needs.
* Add local storage recommendation for monitoring node.
* Better align CPU/Memory to match common cloud provider instnace types.
* Update requirements based on the new 10k user standard.

Updating redirected links in the development, install, policy
raketasks and university dirs. Should now link to final
URL to avoid redirect hops

The installation link has changed from /installation/ to /install/. This
commit will update all the remaining links that are missing.

Now MySQL is no longer supported, we need to change the docs

Also another factor of consideration for
runner server requirements.

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

Updated documentation as per latest missing features on about.gitlab.com

See merge request gitlab-org/gitlab-ee!5794