UX bundle: add information about AssetMapper config

[jmsche]

The UX bundle docs are missing some information about AssetMapper/Import maps.

What I found for now:

• add a "importmaps" config in the "symfony" object, which seems to be the same as the "peerDependencies" config
• configure the bundle to prepend the bundle's asset dist path to the AssetMapper paths
• maybe there's something wrong in my TS config (which should be the same as the docs), but it does miss a default export, so something should be fixed

[javiereguiluz]

Closing as fixed in #18755.