ci: update README
ci: update README

- Git plugin has been renamed and API changed.
- Removed `$` example - `pair` is easier to use.
- Link to SelfCI and MirageCI examples.

Signed-off-by: Thomas Leonard <ea62ade398c3addd3bcc52285a00e98f3b0c618f@docker.com>

docker/datakit,docker/datakit,docker/datakit