Add Discord information to the How To Contribute page

What does this MR do and why?

We mention the Community Run Discord to customers during conversations [1] sometimes but also in presentations [2]. We have an open issue to implement Discord [3], and even though it is still up for debate, we recently received ownership of the community run Discord [4]. This MR makes clear mention that Discord exists, that it is community run and that GitLab acknowledges its existence but at the same time refrains from promoting its use. Given that we are waiting on a decision to move forward with Slack or not, this is the middle-way to make it a bit more clear where we are today when it comes to where our contributors reside, while at the same time avoiding any form of migration of both users or responsibility.

[1] https://docs.google.com/document/d/15j6ZtlEJ8flgMFZjEgSpsL-2dsDFtcOGwV50zAT8ZPA/edit# [2] https://docs.google.com/presentation/d/1FEXx6WeOxMTwQR8hcjBRHm9Usdb6Ruy4URyAvfkizys/edit#slide=id.g14823b9ef88_0_222 [3] https://gitlab.com/gitlab-com/marketing/community-relations/evangelist-program/general/-/issues/1269 [4] gitlab-community-discord/community-issue-tracker#1 (closed)

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