Update doc release duties for TW Lead

Why is this change being made?

As per recent changes to the documentation release process, the TW Lead is not responsible for the docs release process. This is now done by the TW who was TW Lead for the previous release.

This MR updates the release post handbook page and release post template:

• In the TW Lead section in the Release Post handbook page, updates:
  • From list of responsibilities, removes Releasing the next version of the documentation.
  • Versioned documentation release so that it describes the current docs release process.
• Removes docs release task from Structural check in the RP template.

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.

---