This updates our build images to use node@12. We are also adding a
sentence to the from-source installation that while we require a minimum
version of node@8, we also recommend node@12. Last but not least, we
change the version of node in the .nvmrc so that developers using nvm
can automatically use node@12 with the project

Improve details about acceptable number of unicorn workers

See merge request gitlab-org/gitlab-ce!28262

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

Add docs to describe how to compile git with libpcre2

See merge request gitlab-org/gitlab-ce!28199