Add HA architecture examples

This new HA 'Architecture' doc page intends to share some of the potential options for deploying HA:

• 'Horizontal' - Our de facto standard for HA. Most customers use this form of HA
• 'Hybrid' - A halfway point between 'Horizontal' and 'Fully Split' (GitLab.com)
• 'Fully Split - Everything on dedicated nodes like GitLab.com is
• Is there another to consider? There are certainly 'in-betweens' stages of each example, which I think is fine.

@stanhu @lbot Do you see something like this as helpful? Obviously we need some other docs updated to go along with this, but this will help as Sales and Support have conversations with customers.

I'm also considering to put some anonymized customer examples showing how many nodes, what component each run, and the RAM/CPU of each.

What do you think?

closes #5368 (closed)