docs: Move and update Background page

docs/background.md

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+---
+nav_order: 2
+---
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+# Background
+{: .no_toc }
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+1. TOC
+{:toc}
+
 ## Package systems versus image systems
 
 Broadly speaking, software update systems for operating systems tend
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 ### Package system benefits and drawbacks
 
-Benefits
+Benefits:
 
  * Highly dynamic, fast access to wide array of software
  * State management in `/etc` and `/var` is well understood
  * Can swap between major/minor system states (`apt-get upgrade` is similar to `apt-get dist-upgrade`)
  * Generally supports any filesystem or partition layout
 
-Drawbacks
+Drawbacks:
 
  * As package set grows, testing becomes combinatorially more expensive
  * Live system mutation, no rollbacks
 
 ### Image benefits and drawbacks
 
-Benefits
+Benefits:
 
  * Ensures all users are running a known state
  * Rollback supported
  * Easier to verify system integrity
 
-Drawbacks
+Drawbacks:
 
  * Many image systems have a read-only `/etc`, and writable partitions elsewhere
  * Must reboot for updates