[AssetMapper] Deleting duplicated info

[ThomasLandauer]

Page: https://symfony.com/doc/6.4/frontend/asset_mapper.html#deploying-with-the-assetmapper-component

Reason: This is all explained in detail under the very next heading.

[ThomasLandauer]

Plus adding more details about the JSON files (commit 2). If you don't merge this, I'll still open a separate PR with only this part.

[javiereguiluz]

Thanks Thomas.

I agree on removing the duplicated text. However, I disagree with some proposed changes:

• We prefer to not list exhaustively internal details like all the JSON files generated by AssetMapper. This could change, so it would make docs harder to maintain. Instead I listed only two of those JSON files and added an etc.
• I readded the mention to the "optimization of importmap" because that's the exact reason why all these files are generated.

See ef2c3b2

Thanks!