Documentation: add a "why" note for the puma/unicorn/sidekiq reload

What does this MR do?

We've had at least one recent support ticket come in where a customer was running multiple application nodes and had errors result from where they did not reload the unicorn/sidekiq processes on all the application nodes. While our single-node and HA docs make this clear that you'll need to do this with upgrades - there's no mention of "why".

This MR adds a brief mention for why a HUP of unicorn/puma and restart of sidekiq is necessary when going through a zero-downtime upgrade

Changing the header title?

• Is it completely necessary to change the title? Changing titles breaks deep linking. If yes, proceed further.
• Move the old heading title to the bottom of the page.
• Change the header tag to h5, #####.

Moving docs to a new location?

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.