Add Discord handbook to workflows & tools main handbook page

Why is this change being made?

I could not find the Discord documentation in the handbook, visiting the community relations workflows and tools handbook. After research, I found a table in the developer evangelism handbook which links the workflows/tools sub page.

The table partially duplicates the SSoT - a long term update should be to merge the dev evangelism handbook table into the workflows and tools handbook. It is generated from the tech_stack.yml though which might make updates and intentions harder.

As a boring solution, I've added the Discord URL into the workflows and tools listing, and added a note for the developer evangelism handbook to keep edits in sync with the workflows and tools handbook.

Note: I have edited the handbook page with VS Code which automatically trimmed trailing whitespaces, hence the bigger MR diff changes.

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
  • The when to get approval handbook section explains the workflow in more detail
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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