chore: clean up

[innocenzi]

This pull request cleans up the repository:

• It moves CONTRIBUTING.md to the root to increase its visibility
• It adds linting using case-police
• It cleans up the "IDE extensions" section
• It cleans up the "Plugins" section
• It cleans up the "Tools" section
• It cleans up the "UI Libraries" section
• It cleans up the "Starters & Themes" section
• It cleans up the "Learning" section

Some items were removed from the list because:

• They were unmaintained since 2+ years,
• They were explicitly deprecated,
• They were dead links,
• They were hijacked (redirect to something unrelated),
• They became paid products.

Paid products have been removed altogether, and free products with paid plans have been marked with an emoji and moved to the bottom of each section.

I also removed non-official plugins from the collapsible "deprecated plugins" section, as I don't think it was that useful, and quite a lot of plugins were deprecated and the list would have added up for no good reason.

[innocenzi]

This could be merged as-is, I could clean up the remaining sections in another pull request later

[joshmanders]

LGTM 👍

[ErwinAI]

I've just noticed Tailscan is no longer on the list. I was not aware this is only a list for free projects.

Also, some remaining entries are still paid-only.

Is there a possibility to reintroduce paid products but clearly indicate them with a 💵 emoji?

[innocenzi]

As much as possible, we want to avoid paid products on this list.

See Josh's comment: #555 (comment)

We only list paid products that area already on the list IF they have a free tier. Other than that no new products that are paid regardless of a freemium/free tier unless they are exceptionally awesome and we'd be stupid not to list them (which is an insanely unique situation)