build: retrieve userstyles from catppuccin/userstyles

[sgoudham]

This PR supersedes #1981.

I've updated the documentation to clarify what userstyles are if people are looking to create their own ports, as well as using @rubyowo's existing code to pull in metadata from the https://github.com/catppuccin/userstyles repository to link our userstyles.

It's worth mentioning that the CI hasn't been updated to trigger rebuilds off changes to the userstyles repository as @nekowinston is interested in doing this work. In addition to this, I was experimenting with the mermaid diagram by trying to include the userstyle steps. Unfortunately, it's not formatted well so I've chosen to omit it for now.

You can see what it currently looks like below:

stateDiagram-v2
  idea: You have an idea for a port
  idea --> choice

  choice: Have you already completed a draft?
  choice --> is_userstyle: Yes
  choice --> request: No

  request: Open a Port Request discussion
  request --> request_picked_up
  
  request_picked_up: You or somebody else works on a draft
  request_picked_up --> request_complete

  request_complete: The draft is completed to your liking
  request_complete --> draft_complete

  is_userstyle: Is this themed via Stylus?
  is_userstyle --> userstyle_review: Yes
  is_userstyle --> draft_complete: No

  userstyle_review: Submit pull request\nto catppuccin/userstyles
  userstyle_review --> review
  
  draft_complete: Create a Port Review issue
  draft_complete --> review

  review: Review period
  review --> port_adoption

  note right of review
    Community feedback
  end note

  port_adoption: Port adoption

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If this mermaid diagram still seems okay, I'd be happy to include in the docs. Let me know what you think @nekowinston

[nekowinston]

LGTM. After the README itself gets removed from the PR, it's good to go.