Add an explanation of "why" to the CI docs

[eteq]

As discussed in Feb 1, 2023 in-person meeting, we want to add some content to the docs that explain the "why" behind the CI machinery. @TheRealZoidberg volunteered to do this

[mgough-970]

Created 'why' document in new branch. Needs to be merged to be populated.

[eteq]

particularly why questions we know we want explained:

• Why are we rendering to a long-living storage branch and creating the html from that instead of doing both things in one go
• Why are we doing the rather complex way of thinking about requirements