Improve doc notes

[driesvints]

This PR allows to handle the new note format for markdown files that GitHub provides. It also provides backwards compatibility for the old docs.

Before, when reading Markdown files on GitHub for our docs you saw these unformatted notes:

But GitHub has since adopted an official syntax for these notes. In laravel/docs#8092 I've updated all notes in the 9.x docs to the new syntax. Which makes them look like this:

Because of the new syntax, we need to adopt a new regex to parse them. That's what this PR does. At the same time I'm filtering on legacy versions to the old docs are unaffected.

Henceforth, the new syntax for writing notes in our docs will be:

> **Note**  
> A general tip

And:

> **Warning**  
> This requires special attention

It's probably best to first merge the docs PR laravel/docs#8092 and then launch this PR with the new formatting. After merging the docs PR, it'll also need to be merged into master so the changes are applied there as well.

[taylorotwell]

@driesvints is it possible to adjust this PR such that it supports both formats in order to eliminate any possibility of breakage?

[driesvints]

@taylorotwell done. I've rewrote the code so it'll check both and format them accordingly.

[driesvints]

Now it's possible to first merge and deploy this PR and then laravel/docs#8092