zet.tlv: refine the initial UX some more

Conundrum:

I want the app to provide some sample data for people to interact with.

However, I also want them to be able to delete it to make it their own.

BUT I'm not yet convinced the app should allow deletion in general.

Besides, deleting stuff robustly and reliably is a whole other level of
programming.

Solution:

Make people edit the app to clear the initial sample zettels.

This has the "beneficial" side-effect of getting them editing apps on
Teliva.

zet.tlv

@@ -1418,3 +1418,50 @@
     >    os.rename(filename, 'zet')
     >  end
     >end
+- __teliva_timestamp:
+    >Thu Feb 10 07:43:39 2022
+  zettels:
+    >-- initial state of the zettels
+    >-- if you came here to clear the zettels,
+    >-- delete everything (ctrl-k and ctrl-u will delete a whole line at a time)
+    >-- until it looks like this:
+    >--
+    >--   zettels = {
+    >--     root='id1',
+    >--     final=1,
+    >--     id1={
+    >--       data='',
+    >--     },
+    >--   }
+    >--
+    >-- I don't yet trust any deletion feature I create to not mess up your data.
+    >-- Besides, this is a good excuse to start making this app your own.
+    >
+    >zettels = {
+    >  root='id1',
+    >  final=5,
+    >  id1={
+    >    data='this is zettel A\n\nit has some text',
+    >    child='id3',
+    >    next='id2',
+    >  },
+    >  id2={
+    >    data='this is a sibling of zettel A at the top level',
+    >    prev='id1',
+    >    next='id5',
+    >  },
+    >  id3={
+    >    data='this is zettel B, a child of A',
+    >    parent='id1',
+    >    next='id4',
+    >  },
+    >  id4={
+    >    data='this is another child of zettel A, a sibling of B',
+    >    parent='id1',
+    >    prev='id3',
+    >  },
+    >  id5={
+    >    data="(To clean up these sample zettels, hit ctrl-u and edit 'zettels')\n\nI don't yet trust any deletion feature I create to not mess up your data.\nBesides, this is a good excuse to start making this app your own.)",
+    >    prev='id2',
+    >  },
+    >}