Create new page - stage adoption & stage expansion definition in TAM handbook section

Sherrod Patchingrequested to merge
spatching-master-patch-25029 into master

Why is this change being made?

I couldn't find a good page to edit - our detail around stage adoption tracking is hidden in the success plans section and really needs to come out to be it's own page. Let's iterate on this initially and then I will bring the hidden content up to this page and build it out some more

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][transparency]
  • Assign this change to the correct DRI
    • If the DRI for the page/s being updated isn’t immediately 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][mr-buddies-slack].
    • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.

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