Una mig guide update

[unatasha8]

Draft of updated Migration guide articles (includes sidebar nav changes). NOT 'ready for review'

Related Issue or Design Document

• DOCS: Migration guide research & planning
  #9211
• https://docs.google.com/document/d/1c6RT02Y8JuZL4InmU7C7F5pUZH56nHOVOXyDoTyMfPA/edit?pli=1&tab=t.0#heading=h.aw5hspe5kjpj
• https://docs.google.com/document/d/15jJD-vb9AM_2OCdAZAUe-9lZzD5EiK6Pgggs-LS0ubw/edit?pli=1&tab=t.0#heading=h.fl2670d0txa3
• https://docs.google.com/document/d/1-vE3wOP4tVF49tlsNCnGshMWNsYkFZ_8zGQadtMcDnM/edit?tab=t.0

Checklist

• I have read the contributing guidelines and signed the CLA.
• I have referenced an issue containing the design document if my change introduces a new feature.
• I have read the security policy.
• I confirm that this pull request does not address a security vulnerability.
  If this pull request addresses a security vulnerability,
  I confirm that I got approval (please contact [email protected]) from the maintainers to push the changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added the necessary documentation within the code base (if appropriate).

Further comments

[unatasha8]

replaced with new PR