Remove not accessible bold/italic message tips in Dev Evangelism social media handbook

Why is this change being made?

Bold/italic unicode text for social media is an experiment to stand out. In a Slack discussion, I learned that screen readers might read this text character-by-character thus not helping with accessibility. ~~This MR adds a note to use this method carefully ~~

Edit: 2023-01-17 after discussion and research in Slack and the MR review in !118095 (comment 1245191962), we agreed to remove the tip from the handbook, not being in line with our accessibility values. I'll also stop experimenting with these kinds of posts on social media.

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.
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  • 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.

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