Removed deprecated docs in the repo

[0x5bfa]

Description

As the title describes, I suggest to remove the deprecated docs below in th /docs.

• ./docs/SECURITY.md
  • The file was older version than the other SECURITY.md in the root dir.
• ./docs/roadmap.md
  • The README's Roadmap section points to a discussion in the WindowsAppSDK repo
• ./docs/winrt-apis-for-desktop.md
  • Now this is documented in Microsoft Learn. I added a link in the README.
• ./docs/winui3_migration.md
  • Now this is documented in Microsoft Learn. I added a link in the README.

Motivation and Context

It's to avoid confusion.

How Has This Been Tested?

N/A

Screenshots (if appropriate):

N/A

[chiaramooney]

Looks reasonable to me. Will update my docs PR after this is merged. Will hold off approval until @michael-hawker can review.

[beth-panx]

@michael-hawker mind taking a look too?

[0x5bfa]

@michael-hawker can you review this PR?

[michael-hawker]

@codendone this is outside the scope of the mirror file, eh?

[Jay-o-Way]

The MS Learn Style Guide, echoing accessibility best practices (WCAG), advises against using vague phrases like "click here" or "go here" in links because they confuse screen readers and users scanning for context; instead, use descriptive text that clearly states the link's destination, like "Create more with Microsoft templates," making links understandable in isolation and improving overall clarity and usability for everyone.

Would you like to tweak this too, while you're at it? 😇