Add migration procedure docs

[mdefende]

Pull Request

Intent

Provides an overview of the imminent Cheaha storage migration and what users should expect from it

Proposed Changes

• Add full page on the migration to the top of the Cheaha Guide nav
• Add announcement to the News page with link to the migration page

I have ensured all of the following for new or changed content:

• [X ] all images have appropriate alt text. Guidelines
• [ X] all images with text have sufficient contrast ratio. Checker
• [ X] all image text is transcribed or described in body text.
• [ X] all technical terms, jargon, and abbreviations are introduced before they are used.

Style Checklist

I have done all of the following:

• [ X] searched for other relevant pages and crosslinked from my content to them.
• [ X] searched for other relevant pages and crosslinked from them to my content.
• [ X] added redirects in mkdocs.yml for any moved headers or pages.
• [ X] defined new technical terms, jargon, and abbreviations in the glossary.
• [ X] searched for existing technical terms, jargon, and abbreviations in the glossary and added crosslinks to them.
• [ X] properly formatted and introduced key branding terms. List here.

[mdefende]

TO-DO:

• add values for total number of files and amount of storage being moved to make it look impressive
• add which of the mainline partition nodes, amd-hdr100, and intel-dcb nodes are being moved at which times during the compute migration phases

[wwarriner]

Thank you for putting this together! Please take a look at the comments I've made.

[jprorama]

I completed my full pass review of the entire document. This is great info and hopefully the recommended changes will improve understanding and help avoid unintentional user confusion.

[jprorama]

I made a few minor changes. They can be made at any time or ignored.

[wwarriner]

A few additional thoughts and comments.

Slack messages determined it's fine in it's current state

[wwarriner]

Great work, thank you!