Add SVG demos (#61)

docs/demos.md

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+Demos taken with [asciinema](https://asciinema.org/).
+(Yes, I have revoked the access token I used when recording these.)
+
+# No-op
+
+A no-op, where the configuration is already in sync with the remote Gitea instance ([raw cast](./no-op.cast)):
+![No-op update demo](./no-op.svg)
+
+
+# Repo creation
+
+Create a repo by adding an appropriate entry in config ([raw cast](./on-creation.cast)):
+![Repo creation demo](./on-creation.svg)
+
+# Repo update
+
+Make a change to that repo by editing some of its fields in the config file ([raw cast](./update.cast)):
+![Repo update demo](./update.svg)
+
+# Repo deletion
+
+Delete the repo by setting its `"deleted"` field to `true` ([raw cast](./deletion.cast)):
+![Repo deletion demo](./deletion.svg)
+