demos: add demos/README.md and run scripts to generate SVGs

PNGs are updated in a descendant commit

demos/README.md

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+# Screenshots with demos and scripts to generate them
+
+The `demo_*.sh` scripts in this directory demo
+various features of `jj`.
+
+The `run_scripts.sh` script can be used to:
+
+- Run them inside a standardized environment instead of the user's local
+  environment.
+- Generate SVG and PNG images for the scripts.
+
+The PNG images in the repo may be slightly older, as they take up more space in
+the repo.
+
+The SVG images have human-readable diffs, but may look different on different
+computers with different fonts installed.
+
+## Running `run_scripts.sh`
+
+This requires ImageMagick and `term-transcript-cli` to be installed. See
+`run_scripts.sh`'s error messages for some more details.
+
+One way to make all the images and check the output is:
+
+```shell
+cd demos
+./run_scripts.sh demo_*.sh |less
+```
+
+### A note on fonts
+
+The exact PNG output depends on the fonts you have installed on your system.
+
+The screenshots are usually generated on a Debian Linux system and use the "Fira
+Code" font. It can be installed with `sudo apt install fonts-firacode`. It seems
+to include all relevant Unicode symbols and be a little bolder and thus more
+readable than the "Liberation Mono" font, which is used if Fira Code is not
+installed (which also works OK). `Consolas` and `SFMono-Regular` are added for
+web viewing.
+
+`convert -list Fonts` will list the fonts ImageMagick is aware of.