TAM handbook main page reorganization

Why is this change being made?

Reorganize areas of the main TAM handbook page to make it more usable, and in line with TAM activities. Replace redundant information with links to the locations where that information is canonically maintained.

These changes are intended to more closely align the TAM handbook with workflows, and make it easier to find the right information.

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
• Assign this change to the correct DRI
  • If the change relates to any part of the project other than updates to content and/or data files (e.g. team.yml) please make sure to ping @gl-static-site-editor in a comment for a review and merge.
  • If the DRI for the page/s being updated isn't immediate clear, then assign it to your manager.
  • 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

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