Update front page/mentions & guidance in Hacker News handbook

Why is this change being made?

Inspired by a Slack discussion about responding to a Hacker News topic, I reviewed the Hacker News handbook page and created this MR to improve the following:

• Fix the channel where front page notifications are posted to (#developer-evangelism)
• Update the Automation section with more details on 1) mentions of keywords 2) front page posts.
  • Remove the keywords from the page. The SSoT is the Zaps documentation where we often change Zap keywords that are tracked.
• Add guidance into the response section, for team members, e.g. where to ask questions, reach out for response coordination, etc.
• Add more direct URLs to Slack channels, and handbooks.

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.
• 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
• 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.

---

FYI @gitlab-de