Handbook: Add Developer Evangelism Tools page

Why is this change being made?

This MR documents all tools used by the Developer Evangelism team, and additional workflows and procedures to get them setup for the team.

• Added Blackmagic.so, Buffer, Grammarly, Owlead, Speakflow
• Moved Buffer from social-media to tools
• Created Thought Leader examples in social-media handbook and cross-linked for owlead campaigns
• Move Keyhole into Tools and update URLs

Once merged, we can continue with the tasks to add the tools to tech_tack.yml. The YAML structure needs a handbook documentation URL provided by this MR.

https://gitlab.com/gitlab-com/marketing/corporate_marketing/corporate-marketing/-/issues/4832+

Author Checklist

• Provided a concise title for the 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 change to the correct DRI(s)
  • 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 in 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.

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