update links in README.md (#33195)

* update links in README.md

* Switch links to use HEAD url

Co-authored-by: Richard Hartmann <RichiH@users.noreply.github.com>

Co-authored-by: Richard Hartmann <RichiH@users.noreply.github.com>
Co-authored-by: Torkel Ödegaard <torkel@grafana.com>

README.md

@@ -30,8 +30,8 @@ The Grafana documentation is available at [grafana.com/docs](https://grafana.com
 
 If you're interested in contributing to the Grafana project:
 
-- Start by reading the [Contributing guide](/CONTRIBUTING.md).
-- Learn how to set up your local environment, in our [Developer guide](/contribute/developer-guide.md).
+- Start by reading the [Contributing guide](https://github.com/grafana/grafana/blob/HEAD/CONTRIBUTING.md).
+- Learn how to set up your local environment, in our [Developer guide](https://github.com/grafana/grafana/blob/HEAD/contribute/developer-guide.md).
 - Explore our [beginner-friendly issues](https://github.com/grafana/grafana/issues?q=is%3Aopen+is%3Aissue+label%3A%22beginner+friendly%22).
 - Look through our [style guide and Storybook](https://developers.grafana.com/ui/latest/index.html).
 
@@ -40,8 +40,8 @@ If you're interested in contributing to the Grafana project:
 - Follow [@grafana on Twitter](https://twitter.com/grafana/).
 - Read and subscribe to the [Grafana blog](https://grafana.com/blog/).
 - If you have a specific question, check out our [discussion forums](https://community.grafana.com/).
-- For general discussions, join us on the [official Slack](http://slack.raintank.io/) team.
+- For general discussions, join us on the [official Slack](https://slack.grafana.com) team.
 
 ## License
 
-Grafana is distributed under [AGPL-3.0-only](LICENSE). For Apache-2.0 exceptions, see [LICENSING.md](LICENSING.md).
+Grafana is distributed under [AGPL-3.0-only](LICENSE). For Apache-2.0 exceptions, see [LICENSING.md](https://github.com/grafana/grafana/blob/HEAD/LICENSING.md).