Document the gotcha for esbuild css plugin

[machour]

An error is thrown when using either filter for onLoad (seems like esbuild is attempting to load it as js even though "main" is pointed to index.css)

We need to document a little gotcha until we find a proper solution, see discussion here: #4130 (comment)

[KingSora]

@machour I think it would be good if we investigate further on this, because isolated tests show that this issue might be related to other plugins / things above esbuild. Opened an issue on esbuilds side and got the confirmation there as well. #4130 (comment)

[machour]

Definitely, but until then, the issue should be properly documented for our users.

Thank you again for your contribution, already using it! 🙏

[machour]

Nobody complained 🤷🏼