Update theme_experiment.json

[bershanskiy]

Changes

1. Mark WebExtension manifest key theme_experiment is experimental, non-standard. (See the article for details).
2. Remove redundant MDN URLs

This is related to #5273, since removes MDN URLs that don't match the regular pattern. If these URLs should be kept, just tell me and I'll drop the second commit (and update #5273 accordingly).

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[rebloor]

@bershanskiy Thanks for this. However, I would ask that we retain the linked URLs. While they may be redundant on the theme_experiment API page, we published the compatibility table on the browser API support page where they are not redundant.

[bershanskiy]

@rebloor

I would ask that we retain the linked URLs.

Thanks for explanation, I reverted that commit.

[rebloor]

Thanks @bershanskiy