Add docs guidance in HA Reference Architectures for less than 3k users

Recently we've had a few queries around smaller than 3k HA Reference Architectures. The 3k is currently the smallest HA reference architecture we offer as to achieve that target the various components need to be separated and setup correctly to ensure HA.

To achieve HA will require an architecture essentially the same makeup as a 3k architecture but there are several tweaks users can make to make it more palatable for smaller numbers of users, for example:

• Reducing node specs
• Combining certain nodes such as GitLab Rails and Sidekiq
• Moving certain components such as Postgres and Redis to PaaS

We should offer this guidance properly in the docs.