Document how asset timestamps are calculated

I really like the fact that Grav can add timestamps to assets, it's the kind of thing that makes your life a lot easier as a developer! However, until just recently, I was under the impression that these timestamps were updated when the timestamps were updated in a manner similar to how the page caching mechanism works. But it looks like this wasn't the case, and timestamps are actually just a random string generated by Grav when the cache is cleared.

It would be great to have the timestamps update automatically, but more than anything, we need some documentation on how the timestamp mechanism works.

Start by reviewing the existing Grav documentation for asset timestamps and cache clearing. Document how timestamps are generated, when they change, and how this differs from page-cache timestamp behavior. Done means a reader can understand the mechanism without assuming timestamps update automatically.

Written by the indexing model from the issue text.