Generating assets is slow

Right now the job takes more than 5 minutes generating metadata. Since every asset is filled sequentially it will only get worse linearly as more assets are added. This is especially noticeable when developing locally, but might also become an issue in GitHub Actions as the quotas are reached.

On a full site redeploy for [example](https://github.com/bevyengine/bevy-website/actions/runs/5913943929/job/16038774443) 8m 20s out of 11m 20s are spent in the asset phase, out of which 3m are compile time and the 5m 20s are running the generation.

The proposed solution is to parallelize metadata requests as much as possible while trying to not hit service throttling limits.

Edit: my bad, this should probably have been opened in the bevy-website repository.

This issue points to the bevy-website repository rather than bevy-assets. Start by reviewing the linked GitHub Actions run and the asset-generation phase, then compare local and CI timings. Done means metadata generation is parallelized without exceeding service throttling limits and the asset phase is measurably faster.

Written by the indexing model from the issue text.