feat (interface): new UDP external tileset sprite for signs

[chaosvolt]

Summary

SUMMARY: Interface "Add a new sprite for f_sign to the UDP external tileset, fixing the DDA lettering on it"

Purpose of change

Apollo over on the BN server pointed out we have a leftover of DDA we can't axe quite so easily:

Describe the solution

Added a new sign sprite to the external tileset.

Also updated the external tileset documentation, including stuff from #3626 that @NobleJake didn't doc.

Describe alternatives you've considered

Take your pick, the rightmost one is the variant currently used:

Testing

1. Checked tile config JSON for syntax and lint errors.
2. Loaded up compiled test build.
3. Spawned in a sign.

Additional context

A terrible secret

[12:02 PM]Chaosvolt: Shrinking actual text down to a 18x9 space really does turn it into just a bunch of lines. XD
[12:03 PM]Chaosvolt: And no one but us will know it was originally in comic sans. :chaos~2:

[chaosvolt]

Had to update the doc as well. :>