doc/start: add Slack invite link

Add a link to the ceph-storage Slack invitation page. Previously the
link went to a plain old "this is the ceph-storage Slack" page that did
not direct the reader to sign up.

Signed-off-by: Zac Dover <[email protected]>

Author: zdover23

doc/start/get-involved.rst

@@ -33,7 +33,7 @@ These are exciting times in the Ceph community! Get involved!
 |                      | ``#cephfs`` dedicated for Ceph developers.      |                                               |
 +----------------------+-------------------------------------------------+-----------------------------------------------+
 | **Slack**            | Since 2023, the Upstream Ceph community has     |                                               |
-|                      | maintained a community in Slack. Ask your Ceph- | https://ceph-storage.slack.com/               |
+|                      | maintained a community in Slack. Ask your Ceph- | - `Ceph Slack Invite`_                        |
 |                      | related questions in Ceph's Slack channels.     |                                               |
 +----------------------+-------------------------------------------------+-----------------------------------------------+
 | **User List**        | Ask and answer user-related questions by        |                                               |
@@ -100,4 +100,4 @@ These are exciting times in the Ceph community! Get involved!
 .. _Blog: http://ceph.com/community/blog/
 .. _Tracker: http://tracker.ceph.com/
 .. _Ceph Source Code: http://github.com/ceph/ceph
-
+.. _Ceph Slack Invite: https://ceph-storage.slack.com/join/shared_invite/zt-2b91em8b6-NQOKhGYReEIrE28OVncnLQ#/shared-invite/email