Rewrite the README and add a recorded demo

Rework the README into the house style: badges, a one-line pitch, an
anchor nav, a commands table, and dedicated sections for clone, pack, and
the native viewer. Every flag and default is checked against the current
binary so the docs match what kage actually does.

Add a demo recorded with ascii-gif. The tape clones example.com, packs it
to a ZIM and to a self-contained binary, and serves it back offline, so
the whole loop reads in one frame. It sits at the top of the README and on
the docs home.

While reviewing the docs, fix the output path everywhere: the default is
$HOME/data/kage, not the kage-out the pages claimed, including a few
fabricated 'done kage-out/...' lines. Document pack, open, and the native
viewer in the release notes.

docs/content/getting-started/quick-start.md

@@ -19,15 +19,15 @@ errors as it goes; the final summary tells you where the mirror landed.
 
 ```
 kage cloning https://example.com
-done kage-out/example.com
+done $HOME/data/kage/example.com
   pages 12   assets 38
-  open kage serve kage-out/example.com
+  open kage serve $HOME/data/kage/example.com
 ```
 
 ## 2. Look at what landed
 
 ```bash
-ls kage-out/example.com
+ls $HOME/data/kage/example.com
 ```
 
 ```
@@ -46,7 +46,7 @@ serve` runs a local static server so everything resolves exactly as it would
 live:
 
 ```bash
-kage serve kage-out/example.com
+kage serve $HOME/data/kage/example.com
 # open http://127.0.0.1:8800
 ```