reference docs: fix broken links and markdown touch-ups

- fix various broken links
- fix incorrect anchors
- updated links for content that moved, so that no
  redirect to the new location is needed.
- touched-up Markdown;
  - re-format some JSON output
  - add blank line betweeen command and command output
  - format "note" blocks

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/reference/commandline/system_df.md

@@ -66,8 +66,9 @@ my-named-vol                                                       0
 * `UNIQUE SIZE` is the amount of space that is only used by a given image
 * `SIZE` is the virtual size of the image, it is the sum of `SHARED SIZE` and `UNIQUE SIZE`
 
-> **Note**: Network information is not shown because it doesn't consume the disk
-> space.
+> **Note**
+>
+> Network information is not shown because it does not consume disk space.
 
 ## Performance