Update Slack to Gitter (#1637)

CONTRIBUTING.md

@@ -17,5 +17,5 @@ You can reach out in the following ways:
 
 * [Discussions](https://github.com/arbor-sim/arbor/discussions). Any questions or remarks regarding using Arbor
 for your research are welcome.
-* [Slack](https://mcnest.slack.com). If you're interested in developing Arbor itself, you can visit our Slack.
+* [Gitter](https://gitter.im/arbor-sim/community). If you're interested in developing Arbor itself, get in touch on Gitter.
 * [Email](mailto:contact@arbor-sim.org).

README.md

@@ -2,6 +2,7 @@
 [![basic ci](https://github.com/arbor-sim/arbor/actions/workflows/basic.yml/badge.svg)](https://github.com/arbor-sim/arbor/actions/workflows/basic.yml)
 [![gitpod](https://img.shields.io/badge/Gitpod-Ready--to--Code-blue?logo=gitpod)](https://gitpod.io/#https://github.com/arbor-sim/arbor)
 [![docs](https://readthedocs.org/projects/arbor/badge/?version=latest)](https://docs.arbor-sim.org/en/latest/)
+[![Gitter](https://badges.gitter.im/Join Chat.svg)](https://gitter.im/arbor-sim/community)
 
 # Arbor Library
 

doc/contrib/index.rst

@@ -152,6 +152,6 @@ You can reach out in the following ways:
 -  `Discussions <https://github.com/arbor-sim/arbor/discussions>`__. Any
    questions or remarks regarding using Arbor for your research are
    welcome.
--  `Slack <https://mcnest.slack.com>`__. If you’re interested in
-   developing Arbor itself, you can visit our Slack.
+-  `Gitter <https://gitter.im/arbor-sim/community>`__. If you’re interested in
+   developing Arbor itself, get in touch on Gitter.
 -  `Email <mailto:contact@arbor-sim.org>`__.

doc/contrib/pr.rst

@@ -18,7 +18,7 @@ the workflow is as follows:
 
    -  You can use labels to categorize your PR. For very large PRs, it
       is likely the reviewer will have many questions and comments. It
-      can be helpful to reach out on our Slack, Discussions or by email
+      can be helpful to reach out on Gitter, Github Discussions or by email
       beforehand if you’ve big things planned for Arbor!
    -  Commit logical units with clear commit messages.
    -  Add a change summary to the PR message. If you remembered to commit