docs: add "The Anatomy of Unleash"

[thomasheartman]

What

This PR

1. adds a new topic document, "The Anatomy of Unleash", which explains how Unleash is built up as a system.
2. It also moves the "topic guides" sidebar entry from position 4 to position 2.
3. Finally, it introduces a new Figure component for the documentation, to be used with images that should be shown with captions.

Why

Referring to the same numbers as mentioned above, here's some background for these changes:

1. We have gotten requests from enterprise users for a way to help new Unleash users understand the system. Together with customer success and customer journey, we agreed that an explanatory guide would be suitable. It aims to give the reader an introduction into what pieces constitute the Unleash system.
2. As part of a discussion, it was suggested to move topic guides higher up to make them more visible. There's a few reasons for this:
   1. New users of Unleash should be able to keep reading about Unleash after the basic introductory material. When left at the bottom, topic guides are often overlooked
   2. As a justification, it was proposed that reference docs are often the last thing you look for, so it makes sense to put that last.
   3. Thinking about a new user's flow, it also makes some sense: first read introductory material, then dive deeper into what Unleash is and what you can use it for, then look for how-to guides if you're stuck, and finally consult the reference material for later.
3. These diagrams aren't necessarily very self-explanatory, so adding a caption makes a lot of sense. We didn't have a component from this previously, so I added one.

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Name	Status	Preview	Comments	Updated
unleash-docs	✅ Ready (Inspect)	Visit Preview	💬 Add your feedback	Oct 19, 2022 at 11:08AM (UTC)
unleash-monorepo-frontend	✅ Ready (Inspect)	Visit Preview	💬 Add your feedback	Oct 19, 2022 at 11:08AM (UTC)