docs: link the plotter by URL, and record the submodule with its .gitignore

Two directions of the same link: the plotter's README now names and links this project,
and this README's link to the plotter was a relative path, which a forge renders as a 404.
It points at the repository now.

The submodule's pointer moves with its .gitignore fix, which is what made this repository
read as having untracked content in `deps/plotter` on every status.

README.md

@@ -2,7 +2,7 @@
 
 A GTK4 oscilloscope-style viewer for a UDPStreamer source: it receives one stream
 with [`deps/udpsclient`](deps/udpsclient), buffers it in two identical large
-circular buffers, and draws it with [`deps/plotter`](deps/plotter).
+circular buffers, and draws it with [`deps/plotter`](https://git.ferrarinas.it/martino/plotter).
 
 `SPECS.md` is the requirement document; this file describes what was built.