Update Transparency value handbook content for readability

Why is this change being made?

I believe the introduction section to the Transparency value could be clearer. Specifically:

• Remove unsupported assertion (We are above average at keeping things confidential that need to be)
• Removed idiom (poker face)
• Refactor to have the opening paragraph support the statement "Be open above as many things as possible." Moved personal behavior to this paragraph. Moved exceptions to a separate paragraph.
• Minor grammatical changes

Crosslinked with the #values Slack channel for visibility: https://gitlab.slack.com/archives/C9VJYTVDE/p1691429656918379

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 post 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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